patch added to fix breaking activeresource unit tests

I've added a patch to fix up some weirdness I was seeing running the
activeresource unit tests. It's in lighthouse at
http://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/1620-fix-for-breaking-activeresource-unit-tests

Basically just due to the tests not explicitly requiring Benchmark.rb
from the local working directory and ending up using an older
incompatible version from gems instead.

Can I get someone to take a look?

cheers
madlep