internal: remove GitHub from the list of broken providers

See #155 for context

Change-Id: I91bd4186465e7489fd9d28f9f9dc2243218ba6ff
Reviewed-on: https://go-review.googlesource.com/14936
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/internal/token.go b/internal/token.go
index a17d79d..75adbe5 100644
--- a/internal/token.go
+++ b/internal/token.go
@@ -92,7 +92,6 @@
 var brokenAuthHeaderProviders = []string{
 	"https://accounts.google.com/",
 	"https://www.googleapis.com/",
-	"https://github.com/",
 	"https://api.instagram.com/",
 	"https://www.douban.com/",
 	"https://api.dropbox.com/",