Get tests passing with frozen-string-literals enabled.

[pat]

These changes ensure that all string literals can be frozen (as per the optional feature in MRI 2.3 and onwards). I would recommend adding the following to your .travis.yml file to ensure regressions aren't introduced:

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

This will add the flag when the tests are run on MRI 2.4 or newer (while the feature was introduced in 2.3, it doesn't seem to work reliably until 2.4). Please note: tests will currently fail when this flag is set unless test-unit is also updated (as noted in test-unit/test-unit#149).

[kou]

Done.

[pat]

Thanks, greatly appreciated :)