Strategies.objects does not merge allOf with own schema

[danielbradburn]

When you have the following schema:

    Derived:
      allOf:
        - type: object
          properties:
            baseProperty:
              type: string
      properties:
        derivedProperty:
          type: string

Strategies.objects will happily generated objects with baseProperty but properties from it's own schema (i.e. derivedProperty) will not be generated. This is because it chooses either the allOf or it's own schema for generation (i.e. for schema in schema.all_of or [schema]). This should be changed to include all_of and it's own schema.