xajax_response: add paramter to addAssign to suppress output

this enables the posibility to check if an addAssign would
be executed and suppress the update. the cache is updated
so that the next call to addAssign will return false.

the idea is that one can check the result of addAssign and
use a special crafted function to do the replacement.

Signed-off-by: Peter Lieven <pl@kamp.de>