fix MRO issue in the class Asset

[llin17]

This pull request aims to resolve the MRO issue described in #191.

[antoinejeannot]

Actually, the MRO issue belongs to the MyModel class, especially its parents class which are called in the wrong order.
It should be:

class MyModel(MyMixin, MyClass):

instead of the other way around:

class MyModel(MyClass, MyMixin):

With this, no need to fix anything in Asset, and the tests pass as well!

I am closing the PR (that has been opened too long), but the discussions remains of course open in the Issues section :)
However, your PR / issue highlighted an actual MRO issue in the tensorflow models section from core!
I will open a PR to fix it right away.

Thanks!