RFC 6749 mandate expires_in to be an integer (https://tools.ietf.org/html/rfc6749#appendix-A.14) but looks like the integer conversion is not done for implicit grant (oauth2-client side).

We can see the current implementation for the oauth2.parameters.parse_token_response: https://github.com/oauthlib/oauthlib/blob/master/oauthlib/oauth2/rfc6749/parameters.py#L367

And the missing conversion code in the parse_implicit_response : https://github.com/oauthlib/oauthlib/blob/master/oauthlib/oauth2/rfc6749/parameters.py#L367