fix add_model not adding required args when all args are required

When using the __init__ method args on a swagger.model to declare fields, if all args were required, then no fields would be added. They would only be added if an optional arg with a default was present. 
example:
```
@swagger.model
class AllRequiredArgs:
    """ No args get added """
    def __init__(self, arg1, arg2, arg3):
        pass

@swagger.model
class SomeOptionalArgs:
    """ All args get added """
    def __init__(self, arg1, arg2, arg3=False):
        pass