(#11901) Fix sort order error in tests for 'keys' and 'values'

(#11901) Fix sort order error in tests for 'keys' and 'values'

Between Ruby 1.8.7 p352 and p357 the way arrays were returned when using
keys and values in Ruby changed, and due to assumption about the
ordering our tests are now failing.

This patch fixes the issue by using the =~ operator matcher in rspec.
This matcher is implemented as RSpec::Matchers::MatchArray and performs
multiset equality matching of arrays.  Order doesn't matter, but
duplicate values do.

This patch also switches @scope instance variables to memoized let
methods for clarity in the code.

Original Author: Ken Barber
Reviewed-by: Nick Lewis
This commit closes GH-29