Serialized $res key

[EmanueleMinotto]

Closes #748

Since $res could be anything, it could also be an array or an object, so it must be serialized to be used for key checking.

[dlondero]

Agree with @EmanueleMinotto. Was about to open an issue for a problem with dateTimeBetween in conjunction with UniqueGenerator: in this case $res is a DateTime object which was ok for the previous implementation (see #491) using in_array but definitely not ok for array_key_exists.

[fzaninotto]

Thanks!

[vlakoff]

I suppose serialize($res) should be moved out of the loop. I had been too fast, please ignore this comment ;)