Name conflict if tracked model has field named "instance"

[shemigon]

Problem

If a tracked model has a field called instance, creating a instance of the model throws

TypeError: Model.__init__() got multiple values for argument 'instance'

See issue #633

Solution

Removed instance argument from the new model init method and used args[0] instead.

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).

[livenson]

Hi, are there any plans to get this change into the upstream? We are being hit with it and wondering about prospective.

[shemigon]

@livenson I'm not a maintainer of the repository. I created this PR a while ago and it's been unnoticed ever since.