FR: support 'octmode'

[r2evans]

In one of my logs I sometimes need to include results from file.info(.). Unfortunately, the returned mode is class octmode and throws the error

jsonlite::toJSON(file.info("does_not_exist"))
# Error: No method asJSON S3 class: octmode

Three ways it might be handled: as.integer, as.character, and paste0("0", .) (a nod towards the 0-prepend for octal in javascript). All have some merit; none of them support perfect round-trip transformation, though contextually the last (paste0) might suggest it.

What are your thoughts on this?