Include default values in PHP and Ruby models

[nickevansuk]

For OpenBookingError subclasses and other cases where the model includes default values, these should be included in the PHP and Ruby libraries, as they are currently in .NET:

.NET Library:
https://github.com/openactive/OpenActive.NET/blob/dc4514c0c3ca1b57a71528d1c46386f2c6200f12/OpenActive.NET/models/IncompleteCustomerDetailsError.cs#L62-L69

PHP Library:
https://github.com/openactive/models-php/blob/785bfc5d0d06e2d4191033833e67f4aa7ec7b265/src/Models/OA/IncompleteCustomerDetailsError.php#L42-L50

Ruby Library:
https://github.com/openactive/models-ruby/blob/e745ea93084c6d9a4bfa1b28f41e85cd172893bc/lib/openactive/models/incomplete_customer_details_error.rb#L3-L24