oauth2: allow users to register broken OAuth2 implementations

Fixes golang/oauth2#111.

Change-Id: Iaea8adb038bcff91b4b468b1a3bdaa5c03d7e8e7
Reviewed-on: https://go-review.googlesource.com/16976
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

internal/token.go

@@ -115,6 +115,10 @@ var brokenAuthHeaderProviders = []string{
 	"https://www.strava.com/oauth/",
 }
 
+func RegisterBrokenAuthHeaderProvider(tokenURL string) {
+	brokenAuthHeaderProviders = append(brokenAuthHeaderProviders, tokenURL)
+}
+
 // providerAuthHeaderWorks reports whether the OAuth2 server identified by the tokenURL
 // implements the OAuth2 spec correctly
 // See https://code.google.com/p/goauth2/issues/detail?id=31 for background.