The previous definition removed double quote characters,
and hence returned invalid JSON, making it unsuitable
for the most common use case: bootstrapping JSON in
a <script> element.
The original definition was at 0ff7a2d,
without indication that the double quote behavior was intentional.
It seems likely that it was simply an oversight after
copy and pasting the definition of html_escape.
Furthermore, since json_escape does not return a HTML-safe
string if not passed one, it is unnecessary for it to escape
characters other than the slash.