Add missing vendor files

vendor/golang.org/x/net/context/ctxhttp/cancelreq.go

@@ -0,0 +1,19 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build go1.5
+
+package ctxhttp
+
+import "net/http"
+
+func canceler(client *http.Client, req *http.Request) func() {
+	// TODO(djd): Respect any existing value of req.Cancel.
+	ch := make(chan struct{})
+	req.Cancel = ch
+
+	return func() {
+		close(ch)
+	}
+}