Add support for Protobuf format response and unit test (#1479)

`Gin` now have the `protobufBinding` function to check the request format, but didn't have a protobuf response function like `c.YAML()`.
In our company [ByteDance](http://bytedance.com/), the largest internet company using golang in China, we use `gin` to transfer __Protobuf__  instead of __Json__, we have to write some internal library to make some wrappers to achieve that, and the code is not elegant. So we really want such a feature.

context.go

@@ -20,6 +20,7 @@ import (
 	"github.com/gin-contrib/sse"
 	"github.com/gin-gonic/gin/binding"
 	"github.com/gin-gonic/gin/render"
+	"github.com/golang/protobuf/proto"
 )
 
 // Content-Type MIME of the most common data formats.
@@ -845,6 +846,11 @@ func (c *Context) Stream(step func(w io.Writer) bool) {
 	}
 }
 
+// ProtoBuf serializes the given struct as ProtoBuf into the response body.
+func (c *Context) ProtoBuf(code int, obj proto.Message) {
+	c.Render(code, render.ProtoBuf{Data: obj})
+}
+
 /************************************/
 /******** CONTENT NEGOTIATION *******/
 /************************************/