internal: add gitter to broken provider list

Per https://developer.gitter.im/docs/authentication#2-gitter-redirects-back-to-your-site ,
both client_id and client_secret are required request parameters.

Change-Id: I64e40be0ab86ec5aea45597b6582e8059d5117cb
GitHub-Last-Rev: d1f5c9ebe6aec8e20e55d085f5e38d58146404e2
GitHub-Pull-Request: golang/oauth2#346
Reviewed-on: https://go-review.googlesource.com/c/150557
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/internal/token.go b/internal/token.go
index 5ab17b9..0f5b989 100644
--- a/internal/token.go
+++ b/internal/token.go
@@ -134,6 +134,7 @@
 	"https://stackoverflow.com/oauth/access_token",
 	"https://account.health.nokia.com",
 	"https://accounts.zoho.com",
+	"https://gitter.im/login/oauth/token",
 }
 
 // brokenAuthHeaderDomains lists broken providers that issue dynamic endpoints.