[RFR] Fixed Serialization of AbstractEntity

Currently the serialisation of `ConnectToken` trigger the serialisation of `ApiUser`.
However the serialisation of `ApiUser` is naive and nothing in particular is done.
This leads to a really big string (~500 Kb in my case) and can lead to potential break (in my case only in production :/) because of the serialisation of `SensioLabs\Connect\Api\Api`.
Moreover there is no advantage of serialising it as it's not usable afterwards (see #15).