Allow required options to be initialised with string keys

[JoelTowell]

🌈

Problem

Thor supports passing initialised options with either symbol or string keys.

MyCounter.new [1, 2], third: 4
MyCounter.new [1, 2], "third" => 4

However, when the option is required, the string-key form raises Thor::RequiredArgumentMissingError.

This also affects invoke when a required option is declared with a string key and passed with a symbol key, as in #614. I checked that reproduction against this branch and it now passes.

Reproduction

The first commit on this branch adds a failing example for this. It can be checked out independently to see the failure.

Solution

Update the required option check in Thor::Options to use a lookup with indifferent access when removing options from @non_assigned_required.