title: AttributeError: 'Account' object has no attribute 'is_staff'
body: <p>I have customized user model, Here is the model:</p>
  
  <pre><code>from django.db import models
  from django.contrib.auth.models import AbstractBaseUser
  from django.contrib.auth.models import BaseUserManager
  
  
  class AccountManager(BaseUserManager):
  
      def create_user(self, username=None, password=None, **kwargs):
          account = self.model(username=username)
  
          account.set_password(password)
          account.is_stuff = False
          account.save()
  
          return account
  
      def create_superuser(self, username, password, **kwargs):
          account = self.create_user(username, password, **kwargs)
          account.is_admin = True
          account.is_staff = True
          account.save()
  
          return account
  
  
  class Account(AbstractBaseUser):
      username = models.CharField(max_length=40, unique=True)
      is_admin = models.BooleanField(default=False)
      created_at = models.DateTimeField(auto_now_add=True)
      updated_at = models.DateTimeField(auto_now=True)
      initialized = models.BooleanField(default=False)
  
      USERNAME_FIELD = 'username'
  
      objects = AccountManager()
  </code></pre>
  
  <p>then when I try to get list of all users it throws <code>AttributeError: 'Account' object has no attribute 'is_staff'</code> . </p>
  
  <pre><code>class GetUserList(ListAPIView):
      permission_classes = (permissions.IsAdminUser,)
      queryset = Account.objects.all()
      serializer_class = AccountSerializer
  </code></pre>