`_resolve_model` should work with unicode strings

### What is the problem / feature ?

The helper function `rest_framework.serializers._resolve_model` did not work correctly with unicode string representations of models.

### How did it get fixed / implemented ?

Changed the check from `type(obj) == str` to `isinstance(obj, six.string_types)`.

*Here is a cute baby animal picture for your troubles...*

![kitten-and-baby-deer_491862](https://cloud.githubusercontent.com/assets/824194/3059397/241802d2-e1eb-11e3-9c35-27ae77b6ea4c.jpg)