Support negative Content-Length in DataFromReader

You can get an http.Response with ContentLength set to -1 (Chunked encoding), so
for DataFromReader to be useful for those we need to support that.

render/reader.go

@@ -21,7 +21,9 @@ type Reader struct {
 // Render (Reader) writes data with custom ContentType and headers.
 func (r Reader) Render(w http.ResponseWriter) (err error) {
 	r.WriteContentType(w)
-	r.Headers["Content-Length"] = strconv.FormatInt(r.ContentLength, 10)
+	if r.ContentLength >= 0 {
+		r.Headers["Content-Length"] = strconv.FormatInt(r.ContentLength, 10)
+	}
 	r.writeHeaders(w, r.Headers)
 	_, err = io.Copy(w, r.Reader)
 	return