1. What version of Go are you using (go version)?
   go1.5
2. What operating system and processor architecture are you using (go env)?
   Linux xxxxxx 3.10.0-327.el7.x86_64 cmd/cgo: fails with gcc 4.4.1 #1 SMP Thu Nov 19 22:10:57 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux

http client POST io.ReadCloser流，如果不显示指定Request.TransferEncoding为chunked，这时io.ReadCloser数据将被分成多个chunked，并且第一个chunked的大小总是为1.
下面是一个例子：
package main
import (
"io/ioutil"
"net/http"
"strings"
)
func main() {
http.Post("http://172.18.1.218/hehe/test.php", "application/x-www-form-urlencoded", ioutil.NopCloser(strings.NewReader("teststring")))
}
tcpdump抓包，request请求包如下：
Flags [P.], seq 1:191, ack 1, win 229, options [nop,nop,TS val 3434923483 ecr 1719817619], length 190
0x0000: 4500 00f2 aa2d 4000 4006 3409 ac12 01d1 E....-@.@.4.....
0x0010: ac12 01da d166 0050 33bd 6ff2 4f4a afc1 .....f.P3.o.OJ..
0x0020: 8018 00e5 5cb4 0000 0101 080a ccbc c5db ...............
0x0030: 6682 5593 504f 5354 202f 6865 6865 2f74 f.U.POST./hehe/t
0x0040: 6573 742e 7068 7020 4854 5450 2f31 2e31 est.php.HTTP/1.1
0x0050: 0d0a 486f 7374 3a20 3137 322e 3138 2e31 ..Host:.172.18.1
0x0060: 2e32 3138 0d0a 5573 6572 2d41 6765 6e74 .218..User-Agent
0x0070: 3a20 476f 2d68 7474 702d 636c 6965 6e74 :.Go-http-client
0x0080: 2f31 2e31 0d0a 5472 616e 7366 6572 2d45 /1.1..Transfer-E
0x0090: 6e63 6f64 696e 673a 2063 6875 6e6b 6564 ncoding:.chunked
0x00a0: 0d0a 436f 6e74 656e 742d 5479 7065 3a20 ..Content-Type:.
0x00b0: 6170 706c 6963 6174 696f 6e2f 782d 7777 application/x-ww
0x00c0: 772d 666f 726d 2d75 726c 656e 636f 6465 w-form-urlencode
0x00d0: 640d 0a41 6363 6570 742d 456e 636f 6469 d..Accept-Encodi
0x00e0: 6e67 3a20 677a 6970 0d0a 0d0a 310d 0a74 ng:.gzip....1..t
0x00f0: 0d0a 0000 0000 0000 0000 0000 0000 0000 ................
0x0100: 0000 ..
Flags [P.], seq 191:205, ack 1, win 229, options [nop,nop,TS val 3434923483 ecr 1719817619], length 14
0x0000: 4500 0042 aa2e 4000 4006 34b8 ac12 01d1 E..B..@.@.4.....
0x0010: ac12 01da d166 0050 33bd 70b0 4f4a afc1 .....f.P3.p.OJ..
0x0020: 8018 00e5 5c04 0000 0101 080a ccbc c5db ...............
0x0030: 6682 5593 390d 0a65 7374 7374 7269 6e67 f.U.9..eststring
0x0040: 0d0a 0000 0000 0000 0000 0000 0000 0000 ................
0x0050: 0000 ..
从上面的抓包数据可以看到，"teststring"被分成两个chunked，第一个chunked长度为1，chunked为"t"，第二个chunked长度为9,chunked为"eststring"

如果POST io.Reader流，Request.TransferEncoding并不会设置为chunked，而是Request.ContentLength被设置为POST数据的长度，如下面这个例子：
package main
import (
"io/ioutil"
"net/http"
"strings"
)
func main() {
http.Post("http://172.18.1.218/hehe/test.php", "application/x-www-form-urlencoded", strings.NewReader("teststring"))
}
tcpdump抓包，request请求包如下：
Flags [P.], seq 1:187, ack 1, win 229, options [nop,nop,TS val 3434923486 ecr 1719817622], length 186
0x0000: 4500 00ee 7c21 4000 4006 6219 ac12 01d1 E...|!@.@.b.....
0x0010: ac12 01da d167 0050 c2a4 b9c3 ba98 6644 .....g.P......fD
0x0020: 8018 00e5 5cb0 0000 0101 080a ccbc c5de ...............
0x0030: 6682 5596 504f 5354 202f 6865 6865 2f74 f.U.POST./hehe/t
0x0040: 6573 742e 7068 7020 4854 5450 2f31 2e31 est.php.HTTP/1.1
0x0050: 0d0a 486f 7374 3a20 3137 322e 3138 2e31 ..Host:.172.18.1
0x0060: 2e32 3138 0d0a 5573 6572 2d41 6765 6e74 .218..User-Agent
0x0070: 3a20 476f 2d68 7474 702d 636c 6965 6e74 :.Go-http-client
0x0080: 2f31 2e31 0d0a 436f 6e74 656e 742d 4c65 /1.1..Content-Le
0x0090: 6e67 7468 3a20 3130 0d0a 436f 6e74 656e ngth:.10..Conten
0x00a0: 742d 5479 7065 3a20 6170 706c 6963 6174 t-Type:.applicat
0x00b0: 696f 6e2f 782d 7777 772d 666f 726d 2d75 ion/x-www-form-u
0x00c0: 726c 656e 636f 6465 640d 0a41 6363 6570 rlencoded..Accep
0x00d0: 742d 456e 636f 6469 6e67 3a20 677a 6970 t-Encoding:.gzip
0x00e0: 0d0a 0d0a 7465 7374 7374 7269 6e67 0000 ....teststring..
0x00f0: 0000 0000 0000 0000 0000 0000 0000 ..............
可以看出，request请求包中有Content-Length header，数据并不是以chunked方式传输的。

如果POST io.ReadCloser流，并显示指定Request.TransferEncoding为chunked，POST的数据并不会被分为两个chunked。如下面例子：
package main
import (
"fmt"
"io/ioutil"
"net/http"
"strings"
)
func main() {
client := &http.Client{
Transport: &http.Transport{
DisableCompression: true,
},
}
req, err := http.NewRequest("POST", "http://172.18.1.218/hehe/test.php", ioutil.NopCloser(strings.NewReader("teststring")))
if err != nil {
fmt.Println(err)
return
}
req.TransferEncoding = []string{"chunked"}
client.Do(req)
}
tcpdump抓包，request请求包如下：
Flags [P.], seq 1:128, ack 1, win 229, options [nop,nop,TS val 3437830591 ecr 154725], length 127
0x0000: 4500 00b3 f410 4000 4006 ea64 ac12 01d1 E.....@.@..d....
0x0010: ac12 01da ee2f 0050 6ebc d5b1 8c5e 75d2 ...../.Pn....^u.
0x0020: 8018 00e5 5c75 0000 0101 080a cce9 21bf ....\u........!.
0x0030: 0002 5c65 504f 5354 202f 6865 6865 2f74 ..\ePOST./hehe/t
0x0040: 6573 742e 7068 7020 4854 5450 2f31 2e31 est.php.HTTP/1.1
0x0050: 0d0a 486f 7374 3a20 3137 322e 3138 2e31 ..Host:.172.18.1
0x0060: 2e32 3138 0d0a 5573 6572 2d41 6765 6e74 .218..User-Agent
0x0070: 3a20 476f 2d68 7474 702d 636c 6965 6e74 :.Go-http-client
0x0080: 2f31 2e31 0d0a 5472 616e 7366 6572 2d45 /1.1..Transfer-E
0x0090: 6e63 6f64 696e 673a 2063 6875 6e6b 6564 ncoding:.chunked
0x00a0: 0d0a 0d0a 610d 0a74 6573 7473 7472 696e ....a..teststrin
0x00b0: 670d 0a00 0000 0000 0000 0000 0000 0000 g...............
0x00c0: 0000 00 ...
可以看出，request POST是以chunked方式发出去，并且chunked没有分片。