Fix the OR logic for the long argument with MerMod objects

Previously the final test was checking if (not long && isMer.y OR isMer.x) - which always triggered the not long case when a MerMod object was present for model x. Thus, you could never get long output from a mediation with a MerMod x model. Now, it tests if (not long && isMer.y OR not long && isMer.x). The updated code gave the correct output in my test case.