Nested fields are not rendered in model's schema

[alvarolopez]

In current HEAD (9b9ad42) the model's schema is not generated correctly if it contains nested models.

Taking into account the following model definition, where I have a nested model defined:

model_links = ns.model('Location', {                                               
    "rel": fields.String(),                                                        
    "href": fields.Url()                                                           
})                                                                                 
                                                                                   
model_meta = ns.model('Model', {                                                   
    'id': fields.String(),                                                         
    'name': fields.String(),                                                       
    'links': fields.List(fields.Nested(model_links))                               
}) 

If I access the model's schema, I get the following:

{'properties': {'id': {'type': 'string'},
                'links': {'items': {'$ref': '#/definitions/Location'},
                          'type': 'array'},
                'name': {'type': 'string'}},
 'type': 'object'}

As you can see, the nested models are not defined, resulting in an invalid schema. What I would expect is the following:

{'definitions': {'Location': {'properties': {'href': {'type': 'string'},
                                             'rel': {'type': 'string'}},
                              'type': 'object'}},
 'properties': {'id': {'type': 'string'},
                'links': {'items': {'$ref': '#/definitions/Location'},
                          'type': 'array'},
                'name': {'type': 'string'}},
 'type': u'object'}

This is basically because the schema is not generated recursively (in models.RawModel._schema()). However, in the swagger.json this is done recursively and it is rendered correctly.