Find workaround for django bug

The following code will make Django use a string representation of a memory view

```python
    def test_update_one_attribute(self):
        """Test django bug."""
        expected = 'bonjour'
        instance = EncryptedTextFieldModelFactory.create(pgp_pub_field=expected)

        instance.pgp_sym_field = 'new value'
        instance.save()

        updated_instance = self.model.objects.get()
        self.assertEqual(updated_instance.pgp_pub_field_decrypted, expected)
```

```bash
Creating test database for alias 'default'...
............F.....
======================================================================
FAIL: test_update_one_attribute (tests.test_fields.TestEncryptedTextFieldModel)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "django-pgcrypto-fields/tests/test_fields.py", line 196, in test_update_one_attribute
    self.assertEqual(updated_instance.pgp_pub_field_decrypted, expected)
AssertionError: '<memory at 0x1035ce5c0>' != 'bonjour'
- <memory at 0x1035ce5c0>
+ bonjour
```
I think this line is [untested](https://travis-ci.org/incuna/django-pgcrypto-fields/builds/43578460).