Extension of the `key?` method to check for existence of multiple keys in a hash.

hash.rb

class Hash
  # Extension of the `key?` method to check for existence of multiple keys in 
  # a hash.
  # 
  # Returns true if all keys matches, false if hash has missing keys.
  # Returns true if hash has more keys than specified in array.
  def keys?(array)
    !array.map { |k| self.key?(k) }.include? false
  end
end