Add keep parameter to dump to copy invalid UTF-8 bytes as-is

[nlohmann]

Add an enumerator keep to error_handler_t to allow to keep the input as-is in case of UTF-8 errors.

Fixes #4552

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[coveralls]

coverage: 99.639%. remained the same when pulling 4ab98c39c3a7aa7ccd727f83f3350a0d1d44ca4c on issue4552-ignore into 663058e7d18241338aec42846d4f77995275ccf6 on develop.

[jordan-hoang]

It looks good to me. I thought you would do json_string === s_kept, but you took it's substring at position 1 instead, I guess dump gives you an extra character in the beginning or something?

[nlohmann]

Yes, dump adds quotes.

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[gregmarr]

@nlohmann Does this still need work?

[nlohmann]

Yes. And clarification.