mailchimp: add MailChimp provider

Added MailChimp endpoints as per documentation:
http://developer.mailchimp.com/documentation/mailchimp/guides/how-to-use-oauth2/

Add MailChimp to brokenAuthHeadersProviders since they require the
client id and secret as query parameters.

Fixes golang#272

Change-Id: I155d0c7e34f13477ac87b225e9a278d79c3a2a44
Reviewed-on: https://go-review.googlesource.com/96635
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: walle

internal/token.go

@@ -103,6 +103,7 @@ var brokenAuthHeaderProviders = []string{
 	"https://api.twitch.tv/",
 	"https://app.box.com/",
 	"https://connect.stripe.com/",
+	"https://login.mailchimp.com/",
 	"https://login.microsoftonline.com/",
 	"https://login.salesforce.com/",
 	"https://login.windows.net",

mailchimp/mailchimp.go

@@ -0,0 +1,17 @@
+// 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 mailchimp provides constants for using OAuth2 to access MailChimp.
+package mailchimp // import "golang.org/x/oauth2/mailchimp"
+
+import (
+	"golang.org/x/oauth2"
+)
+
+// Endpoint is MailChimp's OAuth 2.0 endpoint.
+// See http://developer.mailchimp.com/documentation/mailchimp/guides/how-to-use-oauth2/
+var Endpoint = oauth2.Endpoint{
+	AuthURL:  "https://login.mailchimp.com/oauth2/authorize",
+	TokenURL: "https://login.mailchimp.com/oauth2/token",
+}