BREAKING CHANGE:
- rename RefreshAccessToken to RefreshToken
- RefreshToken returns *oidc.Tokens instead of *oauth2.Token
This change allows the return of the id_token in an explicit manner,
as part of the oidc.Tokens struct.
The return type is now consistent with the CodeExchange function.
When an id_token is returned, it is verified.
In case no id_token was received,
RefreshTokens will not return an error.
As per specifictation:
https://openid.net/specs/openid-connect-core-1_0.html#RefreshTokenResponse
Upon successful validation of the Refresh Token,
the response body is the Token Response of Section 3.1.3.3
except that it might not contain an id_token.
Closes#364
