Fix #198 (#781)

* Add new function to Render interface for writing content type only

* Add support for the new function in Render interface for writing content-type only

* Fix unhandled merge conflict in context_test.go

* Update vendor.json

render/data.go

@@ -11,10 +11,13 @@ type Data struct {
 	Data        []byte
 }
 
-func (r Data) Render(w http.ResponseWriter) error {
-	if len(r.ContentType) > 0 {
-		w.Header()["Content-Type"] = []string{r.ContentType}
-	}
-	w.Write(r.Data)
-	return nil
+// Render (Data) writes data with custom ContentType
+func (r Data) Render(w http.ResponseWriter) (err error) {
+	r.WriteContentType(w)
+	_, err = w.Write(r.Data)
+	return
+}
+
+func (r Data) WriteContentType(w http.ResponseWriter) {
+	writeContentType(w, []string{r.ContentType})
 }