Fix AutoLastModifiedField default cache

[nesaro]

Problem

AutoLastModifiedField default cache persists for the life of the instance of the field

Solution

I copied a simplified version of the django implementation that returns the callable value if the default is a callable

Commandments

• Write PEP8 compliant code.
• Cover it with tests.
• Update CHANGES.rst file to describe the changes, and quote according issue with GH-<issue_number>.
• Pay attention to backward compatibility, or if it breaks it, explain why.
• Update documentation (if relevant).

[nesaro]

The issue seems not to happen if the model is stored in the database; it seems as if there is a second call that to default on write. I updated the test to reflect the condition that makes it fail