text: Title: AttributeError: 'TemplateResponse' object has no attribute 'charset'
  Body: Django==1.7.6
  splinter==0.7.2
  lxml==3.4.2
  
  ```
  # coding=utf-8
  from django.test import LiveServerTestCase
  from django.contrib.auth import get_user_model
  
  User = get_user_model()
  
  from splinter import Browser
  
  
  class AdminTestCase(LiveServerTestCase):
      def setUp(self):
          User.objects.create_superuser(
              username='admin',
              password='admin',
              email='admin@example.com'
          )
          self.splinter = Browser('django')
          super(AdminTestCase, self).setUp()
  
      def test_create_user_splinter(self):
          count_1 = User.objects.all().count()
          self.splinter.visit('%s%s' % (self.live_server_url, "/admin/"))
  
          self.splinter.find_by_css("#id_username").fill("admin")
          self.splinter.find_by_css("#id_password").fill("admin")
  
          button = self.splinter.find_by_css('[type="submit"]')
          self.assertNotEqual(button, None)
  
          button.click()
          self.splinter.visit('%s%s' % (self.live_server_url, "/admin/app/user/add/"))
  
          self.splinter.find_by_css("#id_username").fill("test")
          self.splinter.find_by_css("#id_password1").fill("test")
          self.splinter.find_by_css("#id_password2").fill("test")
  
          self.splinter.find_by_css("#user_form [type=submit]").first.click()
          count_2 = User.objects.all().count()
  
          self.assertEqual(count_1, count_2 - 1)
  ```
  
  ```
  Traceback (most recent call last):
    File "src/app/tests/test_splinter.py", line 23, in test_create_user_splinter
      self.splinter.find_by_css("#id_username").fill("admin")
    File "env/lib/python2.7/site-packages/splinter/driver/djangoclient.py", line 174, in find_by_css
      return self.find_by_xpath(xpath, original_find="css", original_selector=selector)
    File "env/lib/python2.7/site-packages/splinter/driver/djangoclient.py", line 177, in find_by_xpath
      html = self.htmltree
    File "env/lib/python2.7/site-packages/splinter/driver/djangoclient.py", line 144, in htmltree
      self._html = lxml.html.fromstring(self.html)
    File "env/lib/python2.7/site-packages/splinter/driver/djangoclient.py", line 154, in html
      return self._response.content.decode(self._response.charset)
  AttributeError: 'TemplateResponse' object has no attribute 'charset'
  ```
  