mediaType support

[alvassin]

Hello.

Please add mediaType support.
It is allowed to specify mediaType only once at top level, and then omit it in responses body.

Currently this valid raml will fail with error:

pyraml.model.ValidationError: {'body': "test' expected to be <class 'pyraml.entities.RamlBody'> or dict"}

My valid raml:

#%RAML 0.8
title: Test API
baseUri: http://localhost:8001
mediaType: application/json

schemas:
  - test: !include schemas/test.json

/resources:
  get:
    description: Get all resources
    responses:
      200:
        body:
            schema: test