Fix error in lib/httpclient/util.rb when frozen strings are enabled

[JasonBarnabe]

Also fix some of the tests to work with frozen strings. There are still more errors, including https://github.com/sparklemotion/http-cookie/pull/25.

Closes #395

[olleolleolle]

This is a great piece of work.

The RuboCop Performance gem has written a section about "unfreezing string", and it can be instructive, as well provide automation for any "big change" to this PR. https://docs.rubocop.org/projects/performance/en/stable/cops_performance/#performanceunfreezestring

[dvandersluis]

Will this ever be merged?

[jdelStrother]

Anything we can do that would help merge this?