Fix json_quote_str to handle non-ASCII characters

The current code in json_quote_str() only accepts strict printable ASCII
code points (i.e. 32-127), all other code points are dropped from the
JSON output. The code is attempting to drop only non-printable ASCII
characters, but doing a signed comparison of the byte value is also
dropping characters with values >= 128.

This patch uses an unsigned comparison to accept code points 32-255.

Reviewed-by: Carl Worth <cworth@cworth.org> (with some additional
details for commit message).