oauth2: adds sipgate api to brokenAuthHeaderProviders

according to the documentation, client_id and client_secret must be
provided in the token request:

https://api.sipgate.com/doc/#!/authorization/createOauthAccessToken

Change-Id: I4133a1bfc4d2474013e6b716451c98cb93e30da8
Reviewed-on: https://go-review.googlesource.com/43170
Reviewed-by: Jaana Burcu Dogan <jbd@google.com>
diff --git a/internal/token.go b/internal/token.go
index 251a378..7cce374 100644
--- a/internal/token.go
+++ b/internal/token.go
@@ -120,6 +120,7 @@
 	"https://www.wunderlist.com/oauth/",
 	"https://api.patreon.com/",
 	"https://sandbox.codeswholesale.com/oauth/token",
+	"https://api.sipgate.com/v1/authorization/oauth",
 }
 
 // brokenAuthHeaderDomains lists broken providers that issue dynamic endpoints.