preset ContentLength in first http.Request (#48)

`http.NewRequest(url, method, body)` usually handles req.CL, but we must calculate it since body is initially passed as nil for cloneRequest to handle later on.

Fixes #36.

hey.go

@@ -182,6 +182,7 @@ func main() {
 	if err != nil {
 		usageAndExit(err.Error())
 	}
+	req.ContentLength = int64(len(bodyAll))
 	req.Header = header
 	if username != "" || password != "" {
 		req.SetBasicAuth(username, password)