Fixes frozen string literal bug in engine.rb
What does this MR do and why?
Fix #343 (closed)
We are mutating a string we don't have concrete control over in Engine.rb, when this string is Frozen, engine.rb will fail:
2) Gitlab::Triage::Engine#perform with rules when label in actions:labels does not exist raises Gitlab::Triage::Resource::Label::LabelDoesntExistError
Failure/Error: expect { subject.perform }.to output.to_stdout.and raise_error(Gitlab::Triage::Resource::Label::LabelDoesntExistError)
expected Gitlab::Triage::Resource::Label::LabelDoesntExistError, got #<FrozenError: can't modify frozen String: "Hello"> with backtrace:
# ./lib/gitlab/triage/engine.rb:494:in `tr!'
# ./lib/gitlab/triage/engine.rb:494:in `block in sanitize_resources'