[ruby/json] Stop prebuilding array_delim

The purpose of this change is to exploit `fbuffer_append_char` that is
faster than `fbuffer_append`.

`array_delim` was a buffer that concatenated a single comma with
`array_nl`. However, in the typical use case (`JSON.generate(data)`),
`array_nl` is empty. This means that `array_delim` was a
single-character buffer in many cases.

`fbuffer_append(buffer, array_delim)` used `memcpy` to copy one byte,
which was not so efficient.
Rather, this change uses `fbuffer_append_char(buffer, ',')` and then
`fbuffer_append(buffer, array_nl)` only when `array_nl` is not NULL.

This speeds up `JSON.generate` by about 9% in a benchmark.

https://github.com/ruby/json/commit/445de6e459

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