[1.6.x] Fixed #20961 -- Fixed HttpResponse default empty content

Thanks epandurski at gmail.com for the report.
Backport of f4e980456 from master.

django/http/response.py

@@ -323,7 +323,7 @@ class HttpResponse(HttpResponseBase):
 
     streaming = False
 
-    def __init__(self, content='', *args, **kwargs):
+    def __init__(self, content=b'', *args, **kwargs):
         super(HttpResponse, self).__init__(*args, **kwargs)
         # Content is a bytestring. See the `content` property methods.
         self.content = content

tests/httpwrappers/tests.py

@@ -398,6 +398,13 @@ class HttpResponseTests(unittest.TestCase):
         self.assertEqual(r.tell(), 6)
         self.assertEqual(r.content, b'abcdef')
 
+        # with Content-Encoding header
+        r = HttpResponse()
+        r['Content-Encoding'] = 'winning'
+        r.write(b'abc')
+        r.write(b'def')
+        self.assertEqual(r.content, b'abcdef')
+
     def test_unsafe_redirect(self):
         bad_urls = [
             'data:text/html,<script>window.alert("xss")</script>',