OAuth access token is renewed everytime its retrieved from the store

When trying to retrieve the access token through the `OAuth2ClientScriptService` a new one is returned each time. This suggests that a new call to the OAuth2 provider is made with each request.

To reproduce the issue:

1. Configure a OAuth2 client in XWiki.
2. Authorize the client to the provider using `OAuth2ClientScriptService#authorize(cfgName, redirect)`
3. Call `OAuth2ClientScriptService#getAccessToken(cfgName)` twice

Excepted:

The same access token should be provided

Actual:

Two different access tokens are provided