Fix Hash#slice behavior not to return a subclass instance

[amatsuda]

Hash#slice has been introduced to Ruby core Hash class in 2.5. ruby/ruby@6c50bdda

The Ruby core version of Class(Hash)#slice returns a new instance of Hash, not an instance of the subclass, respecting consistency with other existing Hash methods, e. g.

Class.new(Hash)[:a, 1, :b, 2].select { true }.class
#=> Hash

Class.new(Hash)[:a, 1, :b, 2].reject { false }.class
#=> Hash

This PR fixes the I18n original Hash#slice implementation to behave in the same way with that of Ruby 2.5+ simply by reverting 6f291c4.

I do understand @jimmycuadra's use case described in #250, but in such case, please consider overriding slice inside that particular Hash subclass, just as we're doing in Active Support's hash_with_indifferent_access.rb.

[radar]

Thank you @amatsuda! I prefer your suggestion for an alternative fix to #250. Thank you for including that.