Updates for frozen string literal compatibility.

[pat]

To be clear, @twalpole did all the hard work previously in #1023, so I've doubled up on their efforts (taking a bit of a different approach, mostly to use dup given that seems to be the preference). I've not gone and added all the pragma comments to the files though.

Given that all the testing dependencies seem to be frozen-string-literal friendly, you could take the approach of enabling the flag for MRI 2.4 onwards in the Travis configuration to avoid regressions:

before_script:
- if (ruby -e "exit RUBY_VERSION.to_f >= 2.4"); then export RUBYOPT="--enable-frozen-string-literal"; fi; echo $RUBYOPT

Or adding the pragma comments will do the trick as well.

[schneems]

Is there a game plan for adding the pragma to rack? #1203

[pat]

@schneems I've generally left that up to the maintainers preference, but I'm happy to put the work in and get it in all the files (in this PR or a separate one).

[twalpole]

@pat Without commitment that it will actually get merged, I'd suggest you wait on putting in the work - been there done that (as you pointed out originally)