class-fieldmanager-context-storable.php doesn't pass previous value on update.

[Travis Weston]

Ran into a bug today where previous value is never set when updating post meta.

Tracked the culprit down to this line:

https://github.com/alleyinteractive/wordpress-fieldmanager/blob/master/php/context/class-fieldmanager-context-storable.php#L90

Fix appears to be passing $current to update_data

PR incoming