WSGIFilter does not handle combination of shortcut and RHS generator

[tseaver]

In WSGIFilter-0.1dev-r7189, the middleware's 'call' does not handle the following edge case correctly:

• RHS application returns a generator, delaying call to 'replacement_start_response' until first 'app_iter' iteration.

• When iterated, the RHS calls 'replacement_start_response' with a non-200 status (e.g., '302' for a login form).

• WSGIFilter therefore decides to skip filtering, sets the 'shortcutted' flag, and returns the result of calling LHS 'start response'.

• Iteration over 'app_iter' completes, but the 'captured' list has not been populated, and the (unnwanted) call to 'filter_output' raises.

Patch attached, which re-checks the 'shortcutted' flag after iterating.