3.0 merge

Clean up merges.
String key based arrays should be merged with + operator (3-5x faster and usually more correct with falsy values - unifies this across the core code).

Also refs doc block corrections from https://github.com/cakephp/cakephp/pull/3802
@ADmad @dereuromark sorry, wasn't aware of that; I tend to use the [boy scout rule](http://programmer.97things.oreilly.com/wiki/index.php/The_Boy_Scout_Rule) and *when* I change code, do so according to the CS.