Fix: handle deprecation warning - add unary operator to mutable string

[jsugarman]

Add unary operator to mutable string.

This avoids deprecation warnings when running against ruby 3.4.1 and prepares for later versions (4.0) that will default to # frozen_string_literal: true.

Deprecation warning:

gems/puffing-billy-4.0.0/lib/billy/proxy_connection.rb:33: warning: literal string will be frozen in the future (run with --debug-frozen-string-literal for more information)

Example CI with deprecation warnings: https://app.circleci.com/pipelines/github/ministryofjustice/laa-apply-for-legal-aid/32282/workflows/b137f673-cb4e-4f21-a307-86050bde6e87/jobs/168908/parallel-runs/0?filterBy=ALL