Unmarshalling not working properly when the root does not have `type: object`

[Wim-De-Clercq]

Please see 3f82148 for a failing testcase.

The docs (https://swagger.io/docs/specification/data-models/oneof-anyof-allof-not/#allof) have examples like

    Dog:     # "Dog" is a value for the pet_type property (the discriminator value)
      allOf: # Combines the main `Pet` schema with `Dog`-specific properties 
        - $ref: '#/components/schemas/Pet'
        - type: object
          # all other properties specific to a `Dog`
          properties:
            bark:
              type: boolean
            breed:
              type: string
              enum: [Dingo, Husky, Retriever, Shepherd]
    Cat:     # "Cat" is a value for the pet_type property (the discriminator value)
      allOf: # Combines the main `Pet` schema with `Cat`-specific properties 
        - $ref: '#/components/schemas/Pet'
        - type: object
          # all other properties specific to a `Cat`
          properties:
            hunts:
              type: boolean
            age:
              type: integer

So just a schema with allOf, no type: object.
But using such a schema causes some unmarshalling to be skipped, as a date/datetime string would remain a string after spec_validate_body instead of being parsed.

When type: object is used, (like the comment in the yaml in the commit) then the test case succeeds.

[p1c2u]

Unmarshalling mechanism is very simple for now. It determines the type based on top level schema (no allof, anyfo, oneof are taken under consideration)

[p1c2u]

This should be resolved with the latest unmarshaller finders refactor