backward compatible to v2.2

[kta]

Version 2.3 is not backward compatible with features such as parser

Therefore, an error occurs in a framework other than Rails.

https://github.com/bkeepers/dotenv/blob/master/lib/dotenv/parser.rb#L33

Correction example

      def call(string, is_load = false)         # add default value
        new(string, is_load).call
      end
    end

    def initialize(string, is_load = false)     # add default value
      @string = string
      @hash = {}
      @is_load = is_load
    end

dotenv version:
2.3.0

Ruby version:
2.4.2

[jonmagic]

Closed by #338

Maintainers note: Looks like folks use the private API's so we'll have to think about if we want to remain backwards compatible for internal changes or just declare these API's as public?