clients: set ContentLength explicitly

Set ContentLength explicitly for net/http, so the bodies are sent in
"one piece" and not using chunked transfer encoding, when streaming
is off.

Updates #32.

clients.go

@@ -164,6 +164,7 @@ func (c *httpClient) do() (
 
 	if c.body != nil {
 		br := strings.NewReader(*c.body)
+		req.ContentLength = int64(len(*c.body))
 		req.Body = ioutil.NopCloser(br)
 	} else {
 		bs, bserr := c.bodProd()