Oauth2 Client expects "expires_in" to be a number

[roll57]

Describe the bug
The field expires_in from the POST /oauth2/token is a string, whereas some system expect a number (example Webswing oidc module) which is using google-oauth-java-client

To Reproduce

public final class OAuth2AccessTokenResponseMapConverter ... {
	@Override
	public Map<String, String> convert(OAuth2AccessTokenResponse tokenResponse) {
		...
		parameters.put(OAuth2ParameterNames.EXPIRES_IN, String.valueOf(getExpiresIn(tokenResponse)));
		...
	}
}

Expected behavior
The field should be a number, in the rfc6749 we can see that the example is using a number instead of a string

   {
     "access_token":"2YotnFZFEjr1zCsicMWpAA",
     "token_type":"example",
     "expires_in":3600,
     "refresh_token":"tGzv3JOkF0XG5Qx2TlKWIA",
     "example_parameter":"example_value"
   }

[jgrandja]

Thanks for the report @roll57. I logged an issue in Spring Security as this needs to be fixed there. See spring-security#9685.

We might be able to get a temporary fix in here until it's fixed there.