Fix issue #359: properly validate nullable polymorphic models

[macisamuele]

The goal of this review is to address the validation issue that affects polymorphic models that are nullable (via x-nullable field).

In order to better highlight the reported issue I've modified the polymorphic_specs to ensure that the operation (endpoint) used for testing does return an object that is not directly the polymorphic model.

The issue was mostly caused by the fact that discriminator validation was not using the usual safe check around nullability and so in case of instance=None entering bravado_core.swagger20_validator.discriminator_validator was not short circuiting and instead trying to extract the discriminator key from instance (which was none) and so leading to TypeError being thrown.

Adding the null-gate address the issue an so resolves #359

[coveralls]

Coverage increased (+0.001%) to 98.621% when pulling 0841fe3 on macisamuele:maci-issue-359 into 7c0a3bd on Yelp:master.