Tweaked style of Hash#clear spec
Added specs checking that Hash#except doesn't modify receiver
Added an extra test checking that Hash#filter doesn't modify receiver
Added a test checking Hash#flat_map doesn't modify receiver
Added more tests checking Hash#flatten doesn't modify receiver
Test that Hash#invert doesn't modify receiver
Updated style of Hash#merge spec, added tests ensuring it doesn't modify receiver
Check that Hash is immutable even when RubyHash used to initialize is mutated
Check Hash#partition doesn't modify receiver
Extra test ensuring Hash#remove doesn't modify receiver
Tests ensuring Hash#slice doesn't modify receiver
Test checking Hash#to_hash doesn't modify receiver