fixing nilclass error when clearing shared example groups

Problem occurred for me running guard-jruby-rspec on windows.
add spec to ensure saftey, but this wont work.. Also changelog

Conflicts:
	Changelog.md
Merge pull request #1016 from rspec/safely_clear_shared_example_groups_214

Safely clear shared example groups, rebased off 2-14, speced and change logged.