Improve DefaultCasterRepository and DefaultSerializerRepository constructor

[axlon]

This fixes a very minor pet peeve I have with these two classes;

• The caster repository's casters cannot be passed to its constructor
• The serializer repository's serializers parameter is required

This means that if you build them from scratch you cannot use them in the same way, e.g.

$casters = new DefaultCasterRepository();
$serializers = new DefaultSerializerRepository();

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