Allow child type validation in memory

[dannyshaw]

This PR introduces the ability to validate a child type against the parent prior to the child being saved in the database.

With restricted child_types set, django admin would not allow you to add a valid child type because it tries to validate the object before creating it in the db and the polymorphic_ctype_id is not set to be able to answer the question.

This change simply calls the available pre_save_polymorphic() method to set the id in memory prior to the check.

Test added to expose issue.

[codecov]

Codecov Report

Merging #63 into master will increase coverage by 1%.
The diff coverage is 100%.

@@ Coverage Diff @@ ## master #63 +/- ## ======================================= + Coverage 63.84% 64.85% +1%  ======================================= Files 12 12 Lines 780 791 +11 ======================================= + Hits 498 513 +15  + Misses 282 278 -4

Impacted Files	Coverage Δ
polymorphic_tree/tests/test_models.py	100% <100%> (ø)	⬆️
polymorphic_tree/models.py	63.79% <100%> (+3.79%)	⬆️
polymorphic_tree/tests/models.py	100% <100%> (ø)	⬆️

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[vdboor]

That is a great improvement, thanks!