instance_num do not incremented for multiple values from wgRequest

BUG: by link http://discoursedb.org/wiki/Special:FormEdit/Item/Some_Item?Opinion[1][1]=Topic1&Opinion[1][2]=Position1&Opinion[2][1]=Topic2&Opinion[2][2]=Position2
show two "Opinions...", but "Save page" set only one (last) "Opinions..."
This is because that the variable $instance_num always equals 0

Change-Id: I7a691323fd34d1b0483389e7513d1f43a74f6cb1
Merge "instance_num do not incremented for multiple values from wgRequest"