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/xml.go

@@ -16,6 +16,10 @@ type XML struct {
 var xmlContentType = []string{"application/xml; charset=utf-8"}
 
 func (r XML) Render(w http.ResponseWriter) error {
-	writeContentType(w, xmlContentType)
+	r.WriteContentType(w)
 	return xml.NewEncoder(w).Encode(r.Data)
 }
+
+func (r XML) WriteContentType(w http.ResponseWriter) {
+	writeContentType(w, xmlContentType)
+}