Fixed #24240 -- Allowed GZipping a Unicode StreamingHttpResponse

make_bytes() assumed that if the Content-Encoding header is set, then
everything had already been dealt with bytes-wise, but in a streaming
situation this was not necessarily the case.

make_bytes() is only called when necessary when working with a
StreamingHttpResponse iterable, but by that point the middleware has
added the Content-Encoding header and thus make_bytes() tried to call
bytes(value) (and dies). If it had been a normal HttpResponse,
make_bytes() would have been called when the content was set, well
before the middleware set the Content-Encoding header.

This commit removes the special casing when Content-Encoding is set,
allowing unicode strings to be encoded during the iteration before they
are e.g. gzipped. This behaviour was added a long time ago for #4969 and
it doesn't appear to be necessary any more, as everything is correctly
made into bytes at the appropriate places.

Two new tests, to show that supplying non-ASCII characters to a
StreamingHttpResponse works fine normally, and when passed through the
GZip middleware (the latter dies without the change to make_bytes()).
Removes the test with a nonsense Content-Encoding and Unicode input - if
this were to happen, it can still be encoded as bytes fine.

tests/httpwrappers/tests.py

@@ -348,14 +348,6 @@ class HttpResponseTests(unittest.TestCase):
         #'\xde\x9e' == unichr(1950).encode('utf-8')
         self.assertEqual(r.content, b'123\xde\x9e')
 
-        # with Content-Encoding header
-        r = HttpResponse()
-        r['Content-Encoding'] = 'winning'
-        r.content = [b'abc', b'def']
-        self.assertEqual(r.content, b'abcdef')
-        self.assertRaises(TypeError if six.PY3 else UnicodeEncodeError,
-                          setattr, r, 'content', ['\u079e'])
-
         # .content can safely be accessed multiple times.
         r = HttpResponse(iter(['hello', 'world']))
         self.assertEqual(r.content, r.content)
@@ -512,6 +504,14 @@ class StreamingHttpResponseTests(TestCase):
         self.assertEqual(list(r), [b'abc', b'def'])
         self.assertEqual(list(r), [])
 
+        # iterating over Unicode strings still yields bytestring chunks.
+        r.streaming_content = iter(['hello', 'café'])
+        chunks = list(r)
+        # '\xc3\xa9' == unichr(233).encode('utf-8')
+        self.assertEqual(chunks, [b'hello', b'caf\xc3\xa9'])
+        for chunk in chunks:
+            self.assertIsInstance(chunk, six.binary_type)
+
         # streaming responses don't have a `content` attribute.
         self.assertFalse(hasattr(r, 'content'))