Upgrade gin-gonic to latest master, fix compatibility issues

vendor/github.com/gin-gonic/gin/test_helpers.go

@@ -0,0 +1,17 @@
+// Copyright 2017 Manu Martinez-Almeida.  All rights reserved.
+// Use of this source code is governed by a MIT style
+// license that can be found in the LICENSE file.
+
+package gin
+
+import (
+	"net/http"
+)
+
+func CreateTestContext(w http.ResponseWriter) (c *Context, r *Engine) {
+	r = New()
+	c = r.allocateContext()
+	c.reset()
+	c.writermem.reset(w)
+	return
+}