Adds twitch.tv to the list of known OAuth2 providers that do not implement the spec fully.

Change-Id: I4db53ae2d039de3ddf9cb84e4211d2e0c4f6c41f
Reviewed-on: https://go-review.googlesource.com/5180
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>

oauth2.go

@@ -421,7 +421,8 @@ func providerAuthHeaderWorks(tokenURL string) bool {
 		strings.HasPrefix(tokenURL, "https://www.douban.com/") ||
 		strings.HasPrefix(tokenURL, "https://api.dropbox.com/") ||
 		strings.HasPrefix(tokenURL, "https://api.soundcloud.com/") ||
-		strings.HasPrefix(tokenURL, "https://www.linkedin.com/") {
+		strings.HasPrefix(tokenURL, "https://www.linkedin.com/") ||
+		strings.HasPrefix(tokenURL, "https://api.twitch.tv/") {
 		// Some sites fail to implement the OAuth2 spec fully.
 		return false
 	}