Adding fix and regression test for `trust proxy fn` clobbering

As a result of a change made in [express](/visionmedia/express/commit/566720) and [proxy-addr](expressjs/proxy-addr/commit/7a7a7e), our clobbering of app.settings was causing
a problem (`trust proxy fn` was never being set). This fix re-adds the `trust proxy` setting,
post-clobbering, to ensure express does what it needs to do.

We may want to change to iterating over our `express` config entries and `.set`ing them one-by-one
in the future, but this works for now.