Use a whole number of seconds for the OAuth 2 access token lifetime

The RFC stipulates that the expires_in entity-body parameter of the token response is "The lifetime in seconds of the access token."

More precisely, it is defined by expires-in = 1*DIGIT, so it must be a whole number of seconds.

Yet, we are currently using a double value when serializing the "expires_in" field in JSON and this causes some clients to fail reading the token response.

This was discovered while implementing account linking with OAuth 2 for the Nuxeo Google Assistant, see INNOV-21.