Improve request package and add more unit tests

http/request/cookie.go

@@ -0,0 +1,17 @@
+// Copyright 2018 Frédéric Guillot. All rights reserved.
+// Use of this source code is governed by the Apache 2.0
+// license that can be found in the LICENSE file.
+
+package request // import "miniflux.app/http/request"
+
+import "net/http"
+
+// CookieValue returns the cookie value.
+func CookieValue(r *http.Request, name string) string {
+	cookie, err := r.Cookie(name)
+	if err == http.ErrNoCookie {
+		return ""
+	}
+
+	return cookie.Value
+}