title: Django migrate error : TypeError expected string or bytes-like object
body: <p>I'm trying to learn django and error occur in changing models. I tried a lot like default=datetime.datetime.now but I dont know how to fix it.</p>
  
  <p>these are my models</p>
  
  <p>from django.db import models
  import datetime</p>
  
  <pre><code>class Candidate(models.Model):
      name = models.CharField(max_length=10)
      introduction = models.TextField()
      area = models.CharField(max_length=15)
      party_number=models.IntegerField(default=0)
      def __str__(self) :
          return self.name
  
  class Poll(models.Model) :
      start_date = models.DateTimeField()
      end_date = models.DateTimeField()
      area = models.CharField(max_length=15)
  
  class Choice(models.Model) :
      poll = models.ForeignKey(Poll)
      candidate = models.ForeignKey(Candidate)
      votes = models.IntegerField(default=0)
  </code></pre>
  
  <p>when I type commmand : <code>python manage.py migrate</code>, this error occured</p>
  
  <pre><code>Operations to perform:
    Apply all migrations: admin, ang, auth, contenttypes, sessions
  Running migrations:
    Applying ang.0003_poll_end_date...Traceback (most recent call last):
    File "manage.py", line 22, in &lt;module&gt;
      execute_from_command_line(sys.argv)
    File "C:\Python\Python35-32\lib\site-packages\django\core\management\__init__.py", line 367, in execute_from_command_line
      utility.execute()
    File "C:\Python\Python35-32\lib\site-packages\django\core\management\__init__.py", line 359, in execute
      self.fetch_command(subcommand).run_from_argv(self.argv)
    File "C:\Python\Python35-32\lib\site-packages\django\core\management\base.py", line 294, in run_from_argv
      self.execute(*args, **cmd_options)
    File "C:\Python\Python35-32\lib\site-packages\django\core\management\base.py", line 345, in execute
      output = self.handle(*args, **options)
    File "C:\Python\Python35-32\lib\site-packages\django\core\management\commands\migrate.py", line 204, in handle
      fake_initial=fake_initial,
    File "C:\Python\Python35-32\lib\site-packages\django\db\migrations\executor.py", line 115, in migrate
      state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
    File "C:\Python\Python35-32\lib\site-packages\django\db\migrations\executor.py", line 145, in _migrate_all_forwards
      state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
    File "C:\Python\Python35-32\lib\site-packages\django\db\migrations\executor.py", line 244, in apply_migration
      state = migration.apply(state, schema_editor)
    File "C:\Python\Python35-32\lib\site-packages\django\db\migrations\migration.py", line 129, in apply
      operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
    File "C:\Python\Python35-32\lib\site-packages\django\db\migrations\operations\fields.py", line 84, in database_forwards
      field,
    File "C:\Python\Python35-32\lib\site-packages\django\db\backends\sqlite3\schema.py", line 231, in add_field
      self._remake_table(model, create_fields=[field])
    File "C:\Python\Python35-32\lib\site-packages\django\db\backends\sqlite3\schema.py", line 113, in _remake_table
      self.effective_default(field)
    File "C:\Python\Python35-32\lib\site-packages\django\db\backends\base\schema.py", line 221, in effective_default
      default = field.get_db_prep_save(default, self.connection)
    File "C:\Python\Python35-32\lib\site-packages\django\db\models\fields\__init__.py", line 755, in get_db_prep_save
      prepared=False)
    File "C:\Python\Python35-32\lib\site-packages\django\db\models\fields\__init__.py", line 1438, in get_db_prep_value
      value = self.get_prep_value(value)
    File "C:\Python\Python35-32\lib\site-packages\django\db\models\fields\__init__.py", line 1417, in get_prep_value
      value = super(DateTimeField, self).get_prep_value(value)
    File "C:\Python\Python35-32\lib\site-packages\django\db\models\fields\__init__.py", line 1275, in get_prep_value
      return self.to_python(value)
    File "C:\Python\Python35-32\lib\site-packages\django\db\models\fields\__init__.py", line 1378, in to_python
      parsed = parse_datetime(value)
    File "C:\Python\Python35-32\lib\site-packages\django\utils\dateparse.py", line 93, in parse_datetime
      match = datetime_re.match(value)
  TypeError: expected string or bytes-like object
  </code></pre>
  
  <p>Please help me!!</p>