Form POST should be acceptable as an authentication mechanism for oauth token requests

YJ discovered an issue using the following curl command to retrieve OAuth tokens:

curl -X POST -u "<client_id>:<client_secret>" -d "grant_type=password" -d "username=<oauth username>" -d "password=<oauth password>" <oauth server url>/aml-oauth-provider/oauth/token

This should work. It does not due to a problem in our OAuth provider.