Add assert method to check a response for spec conformance

Similarly to the method for checking HTTP requests for spec conformance,
we're adding a method to do the same for HTTP responses. This one is a bit
less exciting because we're testing raw HTTP responses.

daphne/tests/factories.py

@@ -73,8 +73,8 @@ def _build_request(method, path, params=None, headers=None, body=None):
             quoted_path += b'?' + parse.urlencode(params)
 
     request = method.encode('ascii') + b' ' + quoted_path + b" HTTP/1.1\r\n"
-    for k, v in headers:
-        request += k.encode('ascii') + b': ' + v.encode('ascii') + b"\r\n"
+    for name, value in headers:
+        request += header_line(name, value)
 
     request += b'\r\n'
 
@@ -84,6 +84,13 @@ def _build_request(method, path, params=None, headers=None, body=None):
     return request
 
 
+def header_line(name, value):
+    """
+    Given a header name and value, returns the line to use in a HTTP request or response.
+    """
+    return name.encode('ascii') + b': ' + value.encode('ascii') + b"\r\n"
+
+
 def _run_through_daphne(request, channel_name):
     """
     Returns Daphne's channel message for a given request.

daphne/tests/testcases.py

@@ -4,12 +4,13 @@ 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
+import socket
 import unittest
 
+from . import factories
+
 
 class ASGITestCase(unittest.TestCase):
     """
@@ -122,3 +123,27 @@ class ASGITestCase(unittest.TestCase):
             self.assertIsInstance(server_host, six.text_type)
             self.assert_is_ip_address(server_host)
             self.assertIsInstance(server_port, int)
+
+    def assert_valid_http_response_message(self, message, response):
+        self.assertTrue(message)
+        self.assertTrue(response.startswith(b'HTTP'))
+
+        status_code_bytes = six.text_type(message['status']).encode('ascii')
+        self.assertIn(status_code_bytes, response)
+
+        if 'content' in message:
+            self.assertIn(message['content'], response)
+
+        # Check that headers are in the given order.
+        # N.b. HTTP spec only enforces that the order of header values is kept, but
+        # the ASGI spec requires that order of all headers is kept. This code
+        # checks conformance with the stricter ASGI spec.
+        if 'headers' in message:
+            for name, value in message['headers']:
+                expected_header = factories.header_line(name, value)
+                # Daphne or Twisted turn our lower cased header names ('foo-bar') into title
+                # case ('Foo-Bar'). So technically we want to to match that the header name is
+                # present while ignoring casing, and want to ensure the value is present without
+                # altered casing. The approach below does this well enough.
+                self.assertIn(expected_header.lower(), response.lower())
+                self.assertIn(value.encode('ascii'), response)