Make Hamster::Enumerable#group_by_with protected
Make Hamster::Enumerable#group_by_with protected

There is little reason why people would want to call it outside a subclass. Also, it
expects to receive very specific arguments, and if the wrong arguments (say, a mutable
Array) are passed, it will not do anything useful.

jack-nie/hamster,xaviershay/hamster,dzjuck/hamster