jshn: do not collapse whitespace in json_load()

jshn: do not collapse whitespace in json_load()

When running the test case below the $key variable holds "foo bar" instead
of the expected "foo  bar".

-- 8< --
. /usr/share/libubox/jshn.sh
json_init
json_load '{ "key": "foo  bar" }'
json_get_var key key
-- >8 --

Quote the output of the "jshn -r" backtick expression to prevent the shell
from erroneously collapsing whitespace.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>