get_parent does not work as exptected

[lvqier]

1. Parent is an empty dict:

   >>> from flask_restplus import Namespace
   >>> ns = Namespace('sample')
   >>> a = ns.model('A', {})
   >>> b = ns.inherit('B', a, {})
   >>> b.get_parent('A')
   Traceback (most recent call last):
   File "<console>", line 1, in <module>
   File "/opt/sample/.tox/dev/lib/python3.5/site-packages/flask_restplus/model.py", line 67, in get_parent
   raise ValueError('Parent ' + name + ' not found')
   ValueError: Parent A not found
   >>> from flask_restplus import fields
   >>> a = ns.model('A', {'a': fields.Integer()})
   >>> b = ns.inherit('B', a, {})
   >>> b.get_parent('A')
   Model([('a', <flask_restplus.fields.Integer object at 0x7fe9c2ba1550>)])

2. Model has multiple parents

   >>> from flask_restplus import fields
   >>> ns = Namespace('sample')
   >>> a = ns.model('A', {'a': fields.Integer()})
   >>> b = ns.inherit('B', a, {})
   >>> b.get_parent('A')
   Model([('a', <flask_restplus.fields.Integer object at 0x7fe9c2ba1550>)])
   >>> c = ns.model('C', {'c': fields.Integer()})
   >>> b = ns.inherit('B', c, a, {})
   >>> b.get_parent('A')
   Traceback (most recent call last):
   File "<console>", line 1, in <module>
   File "/opt/zjxta/.tox/dev/lib/python3.5/site-packages/flask_restplus/model.py", line 64, in get_parent
   found = parent.get_parent(name)
   File "/opt/zjxta/.tox/dev/lib/python3.5/site-packages/flask_restplus/model.py", line 67, in get_parent
   raise ValueError('Parent ' + name + ' not found')
   ValueError: Parent A not found

[arabidopsis]

because the check in model.get_parent is just if found but models are instances of dictionaries... and a dictionary without elements is 'falsy'. So A is not found because it is falsy. The get_parent method should really be something like

    def get_parent(self, name):
        if self.name == name:
            return self
        else:
            for parent in self.__parents__:
                try:
                    return parent.get_parent(name)
                except ValueError:
                  pass
        raise ValueError("Parent " + name + " not found")