BUG/MEDIUM: lua: Fix dumping of stick table entries for STD_T_DICT

BUG/MEDIUM: lua: Fix dumping of stick table entries for STD_T_DICT

The issue can easily be reproduced with "stick on" statement

backend BE_NAME
    stick-table type ip size 1k
    stick on src

and calling dump() method on BE_NAME stick table from Lua

Before the fix, HAProxy would return 500 and log something like
the following:
  runtime error: attempt to index a string value from [C] method 'dump'

Where one would expect a Lua table like this:

{
    ["IP_ADDR"] = {
        ["server_id"] = 1,
        ["server_name"] = "srv1"
    }
}

This patch needs to backported to 1.9 and later releases.