feat(request,response) add __dict__ to __slots__ (#868)

Add `__dict__` to the `__slots__` methods to the Request and Response classes
to make it them extensible via subclassing and adding custom attributes. I have
also added tests to make sure adding custom attributes do not break and raise
an `AttributeError`.

Fixes #785

falcon/request.py

@@ -243,6 +243,7 @@ class Request(object):
         'options',
         '_cookies',
         '_cached_access_route',
+        '__dict__',
     )
 
     # Child classes may override this

falcon/response.py

@@ -116,6 +116,7 @@ class Response(object):
         'stream',
         'stream_len',
         'context',
+        '__dict__',
     )
 
     # Child classes may override this

tests/test_slots.py

@@ -0,0 +1,22 @@
+from falcon import Request, Response
+import falcon.testing as testing
+
+
+class TestSlots(testing.TestBase):
+
+    def test_slots_request(self):
+        env = testing.create_environ()
+        req = Request(env)
+
+        try:
+            req.doesnt = 'exist'
+        except AttributeError:
+            self.fail('Unable to add additional variables dynamically')
+
+    def test_slots_response(self):
+        resp = Response()
+
+        try:
+            resp.doesnt = 'exist'
+        except AttributeError:
+            self.fail('Unable to add additional variables dynamically')