Create a hash with only key/value pairs in receiver and allowed
{ :one => 1, :two => 2, :three => 3 }.only(:one) #=> { :one => 1 }
@param [Array[String, Symbol]] *allowed The hash keys to include.
@return [Hash] A new hash with only the selected keys.
@api public
Convert to Mash. This class has semantics of ActiveSupport‘s HashWithIndifferentAccess and we only have it so that people can write params[:key] instead of params[‘key’].
@return [Mash] This hash as a Mash for string or symbol key access.
[Validate]