Correct return types.

Correct return types both in doc blocks and code if possible.

For methods like etag() I would rather like to simplify the return type from `@return string|null` to just "string" for example, though. Especially where there cannot be any empty valid strings or '0' strings involved.
No its a valid unification - see https://github.com/cakephp/cakephp/blob/3.0-returntypes/src/Routing/Filter/AssetFilter.php#L56 (3 lines further down).