workaround unpicklable _on_close handlers

werkzeug.wrappers.Response._on_close is a list of handlers that will be
invoked after the request is processed*. Calling response.make_sequence()
on a streamed response will typically** add unpickleable callables as
_on_close hooks, making the response unpickleable and hence uncacheable.

I suspect this is a bug in werkzeug, and I know Armin himself is unhappy
with werkzeug wrapper picklability***. The docs would make one expect
Response.freeze() to make the response picklable, regardless of _on_close
hooks. Until I further dig into this, I opted to simply make any
response with non-falsy _on_close to be uncacheable. Better safe than
sorry.

* this might not be 100% precise; afaik, the exact call conditions
  are irrelevant to the issue at hand
** read: in all cases I saw
*** http://lucumr.pocoo.org/2012/4/14/im-doing-http-wrong/