Rack::Test usage incorrect: use extend, not include

[synrg]

To successfully run the tests in my Debian package, I have had to use this patch:

http://anonscm.debian.org/gitweb/?p=pkg-ruby-extras/ruby-rack-flash3.git;a=blob;f=debian/patches/extend_not_include_rack_test_methods.patch;hb=HEAD