Ruby Rogues feedback

Ruby Rogues recently did a [code reading of dotenv](https://www.youtube.com/watch?v=lKmY_0uY86s). It was really interesting. Unless you are one of the 5 people in the world that are as fluent in Ruby as @JEG2, there are definitely some interesting things to learn from it.

Most of their constructive criticism was around the `Dotenv.with` method, which is an internal helper that was added in c11eb70646cd44a58b62fa15205a3ce2336f2331 just to remove some duplication. This attempts to fix some of their gripes.

/cc @JEG2 @cmaxw @dbrady
First they discussed the inept use of `protected` on a class method (it doesn't do anything). This replaces it with a simple comment to say that this is an internal helper method. :sparkles: