JSON.generate: warn or raise on duplicated key

Because both strings and symbols keys are serialized the same,
it always has been possible to generate documents with duplicated
keys:

```ruby
>> puts JSON.generate({ foo: 1, "foo" => 2 })
{"foo":1,"foo":2}
```

This is pretty much always a mistake and can cause various
issues because it's not guaranteed how various JSON parsers
will handle this.

Until now I didn't think it was possible to catch such case without
tanking performance, hence why I only made the parser more strict.

But I finally found a way to check for duplicated keys cheaply enough.

0 files changed, 0 insertions, 0 deletions