encoding/json: coerce invalid UTF-8 to valid UTF-8 during Marshal

encoding/json: coerce invalid UTF-8 to valid UTF-8 during Marshal

In practice, rejecting an entire structure due to a single invalid byte
in a string is just too picky, and too hard to track down.
Be consistent with the bulk of the standard library by converting
invalid UTF-8 into UTF-8 with replacement runes.

R=golang-dev, crawshaw
CC=golang-dev
https://golang.org/cl/11211045