declared(params) strips out default values from optional params before validation

I have noticed that @permitted_params = declared(params) will set to null any optional param values when they have a default value set and they have not been passed in the call to an endpoint.

Ideally I think that optional param values should be included in the params returned by the declared method. But this is not what I am seeing.

In other words. If a param that is optional, that has not been passed in the call to the endpoint, but which has a default value specified in the param validation, in the endpoint the param will be in the params array and it will have the default value. However, when passing params through the declared method the returned array has the optional param but the default value has been lost and instead the value is null.

Start by reproducing the declared(params) behavior described in the issue with an optional parameter that has a default value but is omitted from the endpoint call. Trace the parameter validation and declaration flow to find where the default becomes null. Done means the returned params retain the declared default and regression coverage demonstrates that behavior.