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
The array syntax will be updated en mass later on. There are scripts available for it. No need to nitpick for each one :)