[develop 2.0] ensure composition support is correct

[fehguy]

The support for allOf in model definitions is partially complete--need to ensure that the support follows the design of the 2.0 spec.

See here:

https://github.com/swagger-api/swagger-core/blob/develop_2.0/modules/swagger-core/src/test/scala/CompositionTest.scala

[tomtit]

The CompositionTest unit test is enabled now and checks the presence of the 'allOf' entry in the model definition. What else should be done to resolve the issue?

[tomtit]

The first test for the Human class seems to be incorrect as it accepts Pet as a descendant of Human:

"Pet" : {
    "allOf" : [ {
        "$ref" : "#/definitions/Human"
    }

So, here is an improvement #987 for handling subtypes stricter.