Fix: The OPENID_SCOPES maintained by OAuth2AuthHandlerImpl should have offline_access instead of offline

[xinchao-zhang]

Motivation:

This is to fix the issue 2742. According to OpenID spec, there is no "offline" scope, it should be called "offline_access" instead.

Conformance:

You should have signed the Eclipse Contributor Agreement as explained in https://github.com/eclipse/vert.x/blob/master/CONTRIBUTING.md Please also make sure you adhere to the code style guidelines: https://github.com/vert-x3/wiki/wiki/Vert.x-code-style-guidelines