def only(*keys)
      self.reject { |k,v| !keys.include?(k || k.to_sym) }
    end
def only(*keys)
      self.reject { |k,v| !keys.include?(k || k.to_sym) }
    end
Returns a Hash with only the pairs identified by +keys+.

   { :a => 1, :b => 2, :c => 3 }.only(:a)
   => { :a => 1 }