`make vet` error on go master branch

[thinkerou]

error info:

15.41s$ make vet
go vet github.com/gin-gonic/gin github.com/gin-gonic/gin/binding github.com/gin-gonic/gin/ginS github.com/gin-gonic/gin/internal/json github.com/gin-gonic/gin/render
# github.com/gin-gonic/gin
./utils.go:56: method MarshalXML(e *encoding/xml.Encoder, start encoding/xml.StartElement) error should have signature MarshalXML(*xml.Encoder, xml.StartElement) error
# github.com/gin-gonic/gin/render
render/render_test.go:221: method MarshalXML(e *encoding/xml.Encoder, start encoding/xml.StartElement) error should have signature MarshalXML(*xml.Encoder, xml.StartElement) error
make: *** [vet] Error 2
The command "make vet" exited with 2.

details: https://travis-ci.org/gin-gonic/gin/jobs/454760197

[thinkerou]

ref: golang/go#28792

[thinkerou]

golang/go@24ba631 have fixed!