title: Django Serializer AttributeError: 'unicode' object has no attribute 'isoformat'
body: <p>I'm trying to serialize an instance of a (customized) Django User Model like so:</p>
  
  <p>In models.py:</p>
  
  <pre><code>Class Employee(User):
      company = models.ForeignKey('Company')
      additionalField1
      additionalField2
      etc....
  </code></pre>
  
  <p>In the Employee model above, in addition to the attributes inherited from the User model, I use the following model field types: CharField(), NullBooleanField(), IntegerField(), DateField(), DecimalField()</p>
  
  <p>The code in question:</p>
  
  <pre><code>employee = Employee()
  (snip large amounts of code that sets various attributes for employee)
  serializers.serialize("json", [employee, ])
  </code></pre>
  
  <p>(I have WadOfStuff's Django full serializer plugin installed, btw, in case that matters -- but in this case I believe it should be defaulting to the standard Django serializer because I'm not using any of the full serializer's functionality in this case)</p>
  
  <p>The employee __dict__ (with a few key fields anonymized) right before serialization is attempted:</p>
  
  <pre><code>{'status': u'Act', 'last_name': u'Doe', 'payFrequency': u'Mo', '_state':
   &lt;django.db.models.base.ModelState object at 0x15be890&gt;, 'sex': u'M', 'user_ptr_id': 
  None, 'is_staff': False, 'isRegistered': False, 'hireDate': u'2012-08-01', 'id': None, 
  'date_joined': datetime.datetime(2012, 10, 25, 2, 39, 22, 793015, tzinfo=&lt;UTC&gt;), 
  'city': u'San Francisco', 'first_name': u'John', 'zip': u'94114', u'employmentType': 
  u'FT', 'company_id': 4, 'compType': u'S', 'is_superuser': False, 'state': u'CA', 
  'last_login': datetime.datetime(2012, 10, 25, 2, 39, 22, 792983, tzinfo=&lt;UTC&gt;), 
  'email': '', 'username': 'tu7wwhyskewcpheyoq4lk3i3l', 'address2': '', 'is_active': 
  True, 'phone': '', 'address': u'111 Cherry Lane', 'password': 
  'pbkdf2_sha256$10000$OAlOtfQClAV2$OC9oCe/9P5hjc4nWd1ZW6cY117PmW1pny8J41axr6mM=', 
  'salary': u'10833.00', 'standardHours': None, 'dob': u'1980-04-01', 'socialSecurity': 
  u'555555555', 'middleInitial': '', 'payRate': None}
  </code></pre>
  
  <p>Partial traceback:</p>
  
  <pre><code>File "/usr/lib/python2.6/site-packages/django/core/serializers/__init__.py", l                                                                                                                     ine 98, in serialize
  s.serialize(queryset, **options)
  File "/usr/lib/python2.6/site-packages/wadofstuff/django/serializers/base.py",                                                                                                                      line 52, in serialize
  self.handle_field(obj, field)
  File "/usr/lib/python2.6/site-packages/wadofstuff/django/serializers/python.py                                                                                                                     ", line 71, in handle_field
  self._fields[field.name] = field.value_to_string(obj)
  File "/usr/lib/python2.6/site-packages/django/db/models/fields/__init__.py", l                                                                                                                     ine 722, in value_to_string
  return '' if val is None else val.isoformat()
  AttributeError: 'unicode' object has no attribute 'isoformat'
  </code></pre>
  
  <p>Any ideas what might be causing the error or how I can get serialization working in this case? Presumably there is some kind of attribute that the Serializer doesn't like -- how can I figure out which one?</p>