Fix #3017 for Hash#compare_by_identity and Hash#store

https://github.com/rubinius/rubinius/issues/3017

Test in RubySpec:
https://github.com/rubyspec/rubyspec/pull/274
Hash with compare_by_identity should not copy keys
Merge pull request #3018 from godfat/no-copy-for-compare_by_identity

Fix #3017 for Hash#compare_by_identity and Hash#store