gistfile1.rb

class Hash
  def keeping(keys = [])
    raise 'Please specify at least one key!' if keys.empty?
    strkeys = keys.collect {|k| k.to_s}
    self.clone.delete_if {|k,v| !strkeys.include?(k.to_s)}
  end
end

# Tests!
h = {:kyle => 'Bragger', :patrick => 'Ewing'}
puts h.keeping([:kyle]).keys == [:kyle]

h2 = {:foo => 'bar', :baz => ['lol', 'cats'], :quux => {:a => 'b', :c => 'd'}}
puts h2.keeping([:foo, :baz]).keys == [:foo, :baz]