ensure null objects behave correctly with to_a and to_ary

Conflicts:
	spec/rspec/mocks/mock_spec.rb
1.8.7 has a default implementation on top
more descriptive comment
refactor to case statement
Merge pull request #360 from rspec/revisited_bugfix_for_doubles_in_arrays_214

Ensure null objects behave correctly with to_a and to_ary