def format(f = nil)
      if f.nil?
        default_options[:format]
      else
        parser(Parser) if parser.nil?
        default_options[:format] = f
        validate_format
      end
    end
def format(f = nil)
      if f.nil?
        default_options[:format]
      else
        parser(Parser) if parser.nil?
        default_options[:format] = f
        validate_format
      end
    end
Allows setting the format with which to parse.
 Must be one of the allowed formats ie: json, xml

   class Foo
     include HTTParty
     format :json
   end