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/render.go

@@ -8,6 +8,7 @@ import "net/http"
 
 type Render interface {
 	Render(http.ResponseWriter) error
+	WriteContentType(w http.ResponseWriter)
 }
 
 var (