Replace `toLiquid` by `toArray`

This change is intended to make liquid adoption frictionless in PHP where the `toArray` method in an object is widely used to convert it into an array (even though not yet part of PHP's native `ArrayAccess` class.
Re-instate `toLiquid` - with `toArray` fallback
Merge pull request #2 from jadb/use-php-standard-toArray

Replace `toLiquid` by `toArray`