Issue report

Not specifically setting @context, the models auto generated 3 urls and https://openactive.io/ was not first as the validator requires.

Suggested resolution

serialize($obj, $prettyPrint = false) should accept a parameter for $schemaContext = false, which will add "https://schema.org/" as the first element in the @context array of the root.

The function should also detect if any keys in JSON to be outputted contain the "beta:" prefix, and add "https://openactive.io/ns-beta" as the last element in the @context array if so.