Don't modify a signature when injecting on_delete for ForeignKeys.

Review Request #10334 — Created Nov. 26, 2018 and submitted — Latest diff uploaded

Information

Django Evolution
master
7bfa21f...

Reviewers

When setting a default on_delete attribute for mocked ForeignKeys,
we were modifying the dictionary of field attributes going into the
field's constructor. This resulted in the signature's field attributes
being modified, which broke diffing.

This change simply updates the injector code to create a copy of the
dictionary before modifying it.

Unit tests passed.

Didn't hit any diffing issues with schemas.