Fix from tests with polymorphic serializer

[khillion]

Here are some little fix from my own experience with the package.
I have also added a class ChangeField

[khillion]

It should be now possible to:

• Specify several modification for one date using tuple, list or Generator
• Use ChangeField (e.g, from CharField() to IntegerField() to perform corresponding modification in fields and payload.

Tests:

• Unit test for ChangeField.update()
• Add a new modification ChangeField to 2018-08-02 to have a tuple
• All tests of TestVersionedSerializer actually check for the functionality of the ChangeField