(maint) Avoid rspec examples using variable behavior

Given an object with an attr_accessor and using an example shortcut like so:

subject { metadata.to_hash }

its(:keys) do
  subject.sort.should == %w[ name version author summary license source issues_url project_page dependencies ].sort
end

The value of subject in the example varies between RSpec 2.11 and 2.13.
This commit removes the shortcut in favor of a slightly longer but more
compatible test that explicitly invokes the #keys method and does an
expectation on that.