upd internal toJSON to avoid jsonlite message about keep_vec_names

[r2evans]

Prevent jsonlite's message about keep_vec_names=:

### without this patch
toJSON(list(a=5, b=quantile(1:5, 0.5)))
# Input to asJSON(keep_vec_names=TRUE) is a named vector. In a future version of jsonlite, this option will not be supported, and named vectors will be translated into arrays instead of objects. If you want JSON object output, please use a named list instead. See ?toJSON.
# {"a":5,"b":{"50%":3}} 

### with this patch
toJSON(list(a=5, b=quantile(1:5, 0.5)))
# {"a":5,"b":{"50%":3}} 

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[r2evans]

(closed b/c I committed with the wrong username/email, repushed as #4291)