Detect RFC 8785 duplicate keys that differ only in encoding

[Shaisolaris]

With rfc8785: true, a Hash whose String keys have the same text in different encodings is written with that key twice:

latin1 = "\u00e9".encode(Encoding::ISO_8859_1)
JSON.generate({ latin1 => 1, "\u00e9" => 2 }, rfc8785: true)
# => {"é":1,"é":2}

The two keys are distinct Hash keys, but both are written as the UTF-8 string "é", so the canonical output has a duplicate property name. RFC 8785 output is I-JSON, which does not allow duplicate names.

The sort proc already converts keys to UTF-8 and checks for duplicates when an as_json hook is given. It now does the same when a String or Symbol key is not ASCII or valid UTF-8, even without as_json, so this case raises the usual "detected duplicate key" GeneratorError. Hashes whose keys are all ASCII or UTF-8 still skip the conversion.

Tests: added test_rfc8785_duplicate_keys_in_different_encodings to test/json/json_generator_test.rb. It fails on master and passes with the change. Locally (CRuby 3.3, C extension) the rfc8785 tests and the 11 test files under test/json that I ran pass.

[byroot]

Not very convinced by this. String hash keys not encoded in UTF-8 are supposed to be invalid. We should just raise.