Minor performance optimization on stdlib

Some more hidden pearls I didn't really know of: taken from http://stackoverflow.com/a/3471356/347063

I don't know if it's really worth it, but `ArrayUtils::merge` and `PriorityQueue` are quite final, and they're used quite a lot, recursively, causing hundreds of calls to `is_array`. Also, `ArrayUtils::merge` is almost never used with objects (not in a typical ZF2 app), so I went on and applied this one after checking kcachegrind with a skeleton application first