BacktraceCleaner test makes bad assumption about gems

Currently we test the Rails implementation of BacktraceCleaner to see whether it properly cleans gem traces:

  test "should format installed gems correctly" do
    @backtrace = [ "#{Gem.default_dir}/gems/nosuchgem-1.2.3/lib/foo.rb" ]
    @result = @cleaner.clean(@backtrace)
    assert_equal "nosuchgem (1.2.3) lib/foo.rb", @result[0]
  end

However, when we actually build the list of backtrace filters to use, we do it from the array of gem search paths:

Gem.path.each do |path|
... build filter ...

The problem is that Gem.default_dir is not guaranteed to be on the Gem.path list (see http://rubygems.rubyforge.org/rdoc/Gem.html and look at how the two are build), so the test can fail even though the filters are built properly, depending on your Gem install.

The correct change is to fix the test, because when the app is running it'll get gems from the search path. Attached patch does so by testing on Gem.path[0] rather than Gem.default_dir. Applies cleanly to both 2-3-stable and master (and should be applied to both).