Fix DateField.to_representation to work with Python 2 unicode.

[m1kola]

Hello!

Seems like DateField.to_representation doesn't work with unicode values on Python 2.*, but it can accept str values. I think it should work with unicode too.

Here is example:

from rest_framework import serializers.

class Example(object):
    def __init__(self, created):
        self.created = created

class ExampleSerializer(serializers.Serializer):
    created = serializers.DateTimeField()

example = Example(created=u'2016-01-10')
es = ExampleSerializer(example)
print(es.data)

Traceback:

Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/Users/m1kola/.virtualenvs/drf_dev/lib/python2.7/site-packages/rest_framework/serializers.py", line 503, in data
    ret = super(Serializer, self).data
  File "/Users/m1kola/.virtualenvs/drf_dev/lib/python2.7/site-packages/rest_framework/serializers.py", line 239, in data
    self._data = self.to_representation(self.instance)
  File "/Users/m1kola/.virtualenvs/drf_dev/lib/python2.7/site-packages/rest_framework/serializers.py", line 472, in to_representation
    ret[field.field_name] = field.to_representation(attribute)
  File "/Users/m1kola/.virtualenvs/drf_dev/lib/python2.7/site-packages/rest_framework/fields.py", line 1068, in to_representation
    value = value.isoformat()
AttributeError: 'unicode' object has no attribute 'isoformat'

I can reproduce this behaviour on 3.3.1 and master.

[jpadilla]

I think this is a pretty valid enhancement. We could probably just test for six.string_types. #3809 could probably also do with this.

[xordoquy]

string_types is enough, you don't need the test_type here

[m1kola]

PR was updated according comments.

[xordoquy]

Thanks !