Only apply URI.unescape monkey-patch if detected to be required

[kivikakk]

See #32183. Commit message included here:

We test the failing case we're trying to patch; only if it throws an Exception do we patch.

Currently this will always throw, but upstream Ruby has patched this bug: https://git.io/vAxKB

Tested against current Ruby 2.5 (monkey-patch applied, tests pass) and nightly (monkey-patch not applied, tests pass).

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[pixeltrix]

Looks like this broke static file serving with Rack::File because it now returns a 400 Bad Request because the path returned by unescape is US-ASCII with UTF-8 characters.

It strikes me as odd that the unescape method forces the encoding back to the original since a path is going to be US-ASCII by definition but the unescaped version will not be. It has however been this way since 1.9.2 so slightly confused why it's just a problem now (probably because we've been monkey patching forever 😞)

Unsure whether this needs to be fixed in Rails, Rack or Ruby.