Fixing unregistered nested ordered model bug

When setting `ordered=True` in a Namespace, nested models are not registered with `Swagger` giving a "Could not resolve reference" in Swagger documentation.

Author: andre-c-andersen

flask_restx/swagger.py

@@ -18,7 +18,7 @@
 from werkzeug.routing import parse_rule
 
 from . import fields
-from .model import Model, ModelBase
+from .model import Model, ModelBase, OrderedModel
 from .reqparse import RequestParser
 from .utils import merge, not_none, not_none_sorted
 from ._http import HTTPStatus
@@ -663,7 +663,7 @@ def register_model(self, model):
         if isinstance(specs, ModelBase):
             for parent in specs.__parents__:
                 self.register_model(parent)
-        if isinstance(specs, Model):
+        if isinstance(specs, (Model, OrderedModel)):
             for field in itervalues(specs):
                 self.register_field(field)
         return ref(model)