phpcassa\Schema\DataType\UUIDType::pack() accepts string UUIDs

[mcd-php]

Quite incomplete and 'works for me' implementation of string UUID auto-packing, helps remove UUID object creation from client software.

Beware: submitted by web, may have mouso, do run tests (success-case only).

[thobbs]

Thanks for the pull request! I have some comments which I'll make inline.

[mcd-php]

OK, i will rework it to your comments later, thanks for code review.

• canImport() - inconsistency
• if ( !($value instanceof UUID) && is_string($value) ) { ...
• test for throwing InvalidArgumentException, after implementing it in pack()