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

middleware_test.go

@@ -10,8 +10,8 @@ import (
 
 	"testing"
 
-	"github.com/manucorporat/sse"
 	"github.com/stretchr/testify/assert"
+	"gopkg.in/gin-contrib/sse.v0"
 )
 
 func TestMiddlewareGeneralCase(t *testing.T) {