Check order of header values

I'm in the process of updating the ASGI spec to require that the order
of header values is kept. To match that work, I'm adding matching
assertions to the tests.

The code unfortunately is not as elegant as I'd like, but then it's only
a result of the underlying HTTP spec.

daphne/tests/testcases.py

@@ -2,6 +2,9 @@
 Contains a test case class to allow verifying ASGI messages
 """
 from __future__ import unicode_literals
+
+from collections import defaultdict
+
 import six
 import socket
 from six.moves.urllib import parse
@@ -67,13 +70,22 @@ class ASGITestCase(unittest.TestCase):
         # Assert that query_string is a byte string and still url encoded
         self.assertIsInstance(query_string, six.binary_type)
         self.assertEqual(query_string, parse.urlencode(request_params or []).encode('ascii'))
-        # Current implementation doesn't keep ordering
-        headers = channel_message['headers']
-        expected = {
-            (name.lower().strip().encode('ascii'), value.strip().encode('ascii'))
-            for name, value in (request_headers or [])
-        }
-        self.assertEqual(set(headers), expected)
+
+        # Ordering of header names is not important, but the order of values for a header
+        # name is. To assert whether that order is kept, we transform both the request
+        # headers and the channel message headers into a dictionary
+        # {name: [value1, value2, ...]} and check if they're equal.
+        transformed_message_headers = defaultdict(list)
+        for name, value in channel_message['headers']:
+            transformed_message_headers[name].append(value)
+
+        transformed_request_headers = defaultdict(list)
+        for name, value in (request_headers or []):
+            expected_name = name.lower().strip().encode('ascii')
+            expected_value = value.strip().encode('ascii')
+            transformed_request_headers[expected_name].append(expected_value)
+
+        self.assertEqual(transformed_message_headers, transformed_request_headers)
 
         # == Assertions about optional channel_message fields ==