Fix issue with 'from' clause and frozen_string_literal magic comment

[khataev]

I suggest a fix for situation when adding magic comment frozen_string_literal to file with ROXML definition

  # frozen_string_literal: true

  class Book
    include ROXML

    xml_reader :isbn, from: '@isbn'
  end

having this, you'll get the following error during serialization:

FrozenError: can't modify frozen String: "@isbn"
from /Users/khataev/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/gems/roxml-4.1.0/lib/roxml/definition.rb:63:in `sub!'

fix is simple, replacing bang version of sub! with safe version

[Empact]

This looks good, would you add a test for this?

[khataev]

Shure )

[khataev]

Done

[Empact]

Thanks 👍

[khataev]

@Empact when will we get updated gem with current fix?

[Empact]

Released as v4.1.1: https://github.com/Empact/roxml/releases/tag/v4.1.1