Update test_validate_invalid_polymorphic_does_not_alter_validation_paths assertion

[macisamuele]

The assertion seems to break our internal tests.

>       assert validation_error.message == '\'birth_date\' is a required property'
E       assert "u'birth_date...ired property" == "'birth_date' ...ired property"

This is most likely caused by the usual unicode issue :(

I do see two potential approaches there:

• ensure that from __future__ import unicode_literals is present on all python files (via pre-commit hook)
• invert and relax the assertion

I wanted to try the first approach but it ends up taking a bit longer as a few tests need to be fixed (issue similar to the above snipped), so I'm relaxing the condition first so we could get internal tests green again

[coveralls]

Coverage remained the same at 98.333% when pulling 7bf317eee22997ec7450bc75a54ec85469ce5e26 on macisamuele:maci-update-broken-test into cde813a6b89e2cf8322e0ad737f294955a512bad on Yelp:master.

[macisamuele]

Replaced by #316