add Request.CloneWithBody

Fixes #104

fsthttp/request.go

@@ -210,7 +210,7 @@ func newClientRequest() (*Request, error) {
 }
 
 // Clone returns a copy of the request. The returned copy will have a nil Body
-// field, and it's URL will have a nil User field.
+// field, and its URL will have a nil User field.
 func (req *Request) Clone() *Request {
 	return &Request{
 		Method:                    req.Method,
@@ -230,6 +230,14 @@ func (req *Request) Clone() *Request {
 	}
 }
 
+// CloneWithBody returns a copy of the request, with the Body field set
+// to the provided io.Reader.  Its URL will have a nil User field.
+func (req *Request) CloneWithBody(body io.Reader) *Request {
+	r := req.Clone()
+	r.Body = makeBodyFor(body)
+	return r
+}
+
 func cloneURL(u *url.URL) *url.URL {
 	return &url.URL{
 		Scheme:      u.Scheme,
@@ -560,8 +568,8 @@ func makeBodyFor(r io.Reader) io.ReadCloser {
 		return nil
 	}
 
-	if b, ok := r.(*fastly.HTTPBody); ok {
-		return b
+	if rc, ok := r.(io.ReadCloser); ok {
+		return rc
 	}
 
 	return nopCloser{r}