add sync.Pool to manager JSON,XML,YAML,ProtoBuf,MsgPack Render object.
we can move JSON,XML,YAML,ProtoBuf,MsgPack Render implement to sub-module in Gin or other repository module based on this change.
* 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