Don't monkeypatch pecan.core.Request in newer versions of pecan

An upcoming version of pecan will allow you to specify a custom pecan.request
implementation in the Pecan WSGI application constructor.  Detect support for
this functionality and attempt to utilize it instead of the current
monkeypatching approach.

Change-Id: Ie3c257d0cf0618c20af2b834a9ec9698534d23c5