getAccessToken fails when refreshing a token

[hkwak]

The \OpenIDConnectClient\OpenIDConnectProvider::getAccessToken method is not only used for exchanging the initial token, but also for obtaining the Refresh Token.
When refreshing a token, there is no ID Token included, as there is no authentication.

This is causing the getAccessToken method to fail.
https://github.com/steverhoades/oauth2-openid-connect-client/blob/master/src/OpenIDConnectProvider.php#L130

Because getAccessToken is used for refreshing, I believe the best solution would be to fall back to the standard oauth flow
and just call a parent class if the method is only used for refreshing the token (when the grant is 'refresh_token')

[hkwak]

Proposed solution: #40