models: fix for mysql uuid

[roksys]

• Addresses MySQL not working probably cookiecutter-invenio-instance#184

[kpsherva]

I've investigated the issue a bit and there are my findings:
thebinary=False option will actually cause allocating more resources for the uuids (which might be an issue when we have a lot of records
also I have checked step by step, and uuid is deserialised properly in the UUIDType
the POST is handled correctly
the problem is when we try to hit records/1 in the debug mode -> when the debug sidebar is visible
the object gets into the template of the debug sidebar with still byte string not being encoded

(check pid row)