Improve compatibility with frozen string literal

[Throne3d]

When ActionView::Template.frozen_string_literal is set to true (e.g. via Rails.config.action_view.frozen_string_literal), we get errors while rendering exception_notifier templates inside PP.pp on Ruby 3.3.6:

   ActionView::Template::Error: can't modify frozen String: ""

  /app/vendor/ruby-3.3.6/lib/ruby/3.3.0/prettyprint.rb:184:in `text'
  /app/vendor/ruby-3.3.6/lib/ruby/3.3.0/prettyprint.rb:252:in `group'
  /app/vendor/ruby-3.3.6/lib/ruby/3.3.0/pp.rb:201:in `pp'
  /app/vendor/ruby-3.3.6/lib/ruby/3.3.0/pp.rb:97:in `block in pp'
  /app/vendor/ruby-3.3.6/lib/ruby/3.3.0/pp.rb:158:in `guard_inspect_key'
  /app/vendor/ruby-3.3.6/lib/ruby/3.3.0/pp.rb:97:in `pp'
  /app/vendor/bundle/ruby/3.3.0/gems/exception_notification-4.5.0/lib/exception_notifier/views/exception_notifier/_session.text.erb:2:in `_vendor_bundle_ruby_______gems_exception_notification_______lib_exception_notifier_views_exception_notifier__session_text_erb__2544461215938986440_61040'

The emails still come through, but the session component is replaced with this exception instead. I've seen other libraries avoid this for strings that need to be mutable by replacing "" with +"", to ensure it's a copy of the literal and therefore mutable.

I'm not entirely sure this entirely solves the problem - I wasn't able to properly run the exception_notification tests locally - but it should help out! In my application, this fixes the session section, but I haven't tested data thoroughly.

I understand Ruby 3.4 will make this configuration the default, so this may become more pressing shortly!