oauth2/internal: Add api.dropboxapi.com to broken providers.

This is actually the recommended endpoint per the API docs:
https://www.dropbox.com/developers/documentation/http/documentation

Change-Id: I320189c94f727d10a75c4fde4768cef30c28030f
Reviewed-on: https://go-review.googlesource.com/21143
Reviewed-by: Brad Fitzpatrick <[email protected]>

Author: diwakergupta

internal/token.go

@@ -92,6 +92,7 @@ func (e *expirationTime) UnmarshalJSON(b []byte) error {
 var brokenAuthHeaderProviders = []string{
 	"https://accounts.google.com/",
 	"https://api.dropbox.com/",
+	"https://api.dropboxapi.com/",
 	"https://api.instagram.com/",
 	"https://api.netatmo.net/",
 	"https://api.odnoklassniki.ru/",