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>

sh/jshn.sh

@@ -174,7 +174,7 @@ json_add_double() {
 # functions read access to json variables
 
 json_load() {
-	eval `jshn -r "$1"`
+	eval "`jshn -r "$1"`"
 }
 
 json_dump() {