Moved response type checking to preserve explicit serializer registratio...

...n

This allows to reference explicitly added serializers via
```
serializer: myapp.serializers.CustomSerializer
```
as a model even if a custom response is defined using
```
type:
    myparam:
        description: My custom response param
        required: true
        type: string
```

See question here: https://github.com/marcgibbons/django-rest-swagger/pull/60#issuecomment-44956077

This does not affect any logic and tests pass.