diff --git a/vendor/golang.org/x/net/http/httpguts/guts.go b/vendor/golang.org/x/net/http/httpguts/guts.go
new file mode 100644
index 0000000..e6cd0ce
--- /dev/null
+++ b/vendor/golang.org/x/net/http/httpguts/guts.go
@@ -0,0 +1,50 @@
+// Copyright 2018 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.
+
+// Package httpguts provides functions implementing various details
+// of the HTTP specification.
+//
+// This package is shared by the standard library (which vendors it)
+// and x/net/http2. It comes with no API stability promise.
+package httpguts
+
+import (
+	"net/textproto"
+	"strings"
+)
+
+// ValidTrailerHeader reports whether name is a valid header field name to appear
+// in trailers.
+// See RFC 7230, Section 4.1.2
+func ValidTrailerHeader(name string) bool {
+	name = textproto.CanonicalMIMEHeaderKey(name)
+	if strings.HasPrefix(name, "If-") || badTrailer[name] {
+		return false
+	}
+	return true
+}
+
+var badTrailer = map[string]bool{
+	"Authorization":       true,
+	"Cache-Control":       true,
+	"Connection":          true,
+	"Content-Encoding":    true,
+	"Content-Length":      true,
+	"Content-Range":       true,
+	"Content-Type":        true,
+	"Expect":              true,
+	"Host":                true,
+	"Keep-Alive":          true,
+	"Max-Forwards":        true,
+	"Pragma":              true,
+	"Proxy-Authenticate":  true,
+	"Proxy-Authorization": true,
+	"Proxy-Connection":    true,
+	"Range":               true,
+	"Realm":               true,
+	"Te":                  true,
+	"Trailer":             true,
+	"Transfer-Encoding":   true,
+	"Www-Authenticate":    true,
+}