Fix assertion in test_merge_cookies_resp_is_wsgi_callable

As pointed out by @merwok, this assertion didn't make much sense.  This
changes it so the `Set-Cookie` header is actually checked.

Pylons#466 (comment)

Author: luhn

tests/test_response.py

@@ -1052,7 +1052,7 @@ def dummy_wsgi_callable(environ, start_response):
     environ = {}
 
     def dummy_start_response(status, headers, exc_info=None):
-        assert headers, [("Set-Cookie" == "a=1; Path=/")]
+        assert headers == [("Set-Cookie", "a=1; Path=/")]
 
     result = wsgiapp(environ, dummy_start_response)
     assert result == "abc"