fixed FrozenError caused by frozen-string-literal

[taichi-ishitani]

Hi,

I got FrozenError when I used rubyzip gem with --enable-frozen-string-literal option.

FrozenError:
       can't modify frozen String, created at C:/Ruby/Ruby25-x64/lib/ruby/gems/2.5.0/gems/rubyzip-1.2.2/lib/zip/entry.rb:605

This PR is to fix the above error.

[coveralls]

Coverage increased (+0.4%) to 95.751% when pulling 0e6e626 on taichi-ishitani:master into d07b13a on rubyzip:master.

[jdleesmiller]

Thanks!

I'm helping Alex with maintenance now, and I'll get this merged. I think the problem with the CI is that it's testing on some versions of ruby that are now end of life, so I may remove the changes in 0e6e626, but they do help for now.

[jdleesmiller]

Linking this up: #374 also tried to address this. Neither PR is a superset of the other, so I'm not sure either one addressed the issue fully, but it is probably all still helping.

I think the best way forward is to get rubocop up to date and enforce the # frozen_string_literal: true magic comments in all the rubyzip files, so it's not dependent on end user flags.

[taichi-ishitani]

https://github.com/Invoca/magic_frozen_string_literal

I think this gem can help you to add the magic comment.