strappend should remove surrounding ""

[Guy Korland]

127.0.0.1:6379> JSON.SET str . '"foo"'
OK
(5.49s)
127.0.0.1:6379> JSON.Strlen str . 
(integer) 3
127.0.0.1:6379> JSON.STRAPPEND str . '"bar"'
(integer) 8

Should have return

(integer) 6