ArrayUtils:merge() replaced array_key_exists() to isset() for performance

minor performance tweak for ArrayUtils::merge() since array_key_exists() chokes on larger arrays.

here are some performance comparisons and some considerable performance improvement:

a. using isset and &$b
http://3v4l.org/ubuVs

b. using only isset
http://3v4l.org/mTvKJ

c. original array_key_exists
http://3v4l.org/egnKV