Here's something very strange:
- I save the file without the `# encoding: utf-8` (in vim, with `:w`)
- I run `bin/mspec -t x19 spec/ruby/library/cgi/escape_spec.rb`
- It fails (due to `input.valid_encoding?` returning `false`)
- I run again `bin/mspec -t x19 spec/ruby/library/cgi/escape_spec.rb`
- It passes, and does so every time I re-run the specs
- I go back and save the file again (again, just a `:w`)
- It fails the first time (same reason as before) and passes every time after that.

I'm baffled.