Add an optional default parameter for WP_REST_Request::get_param method

[ediamin]

This patch adds a default parameter for WP_REST_Request::get_param method, just like we have a default param for get_option function.

[ediamin]

Proposed changes for WP_REST_Request::get_param method

[TimothyBlynJacobs]

What is the use case for a default value that you wouldn't want to provide by using the JSON Schema default syntax which will automatically fill in a missing value with the default.

[ediamin]

The default property in JSON Schema is not a required property and anyone could skip it.

[TimothyBlynJacobs]

Skip it in what sense? The server/request will apply the default automatically.

In what instances do you know the proper default value for a param, but not in the position to notate that default in the schema.

[TimothyBlynJacobs]

Conditionally adding the close keyword. @ediamin if you could illustrate the scenario you had in mind that'd be great!