Should a default value be returned (by default) for children which are not present?

[notEthan]

Currently JSI returns a default value in place of a nonexistent child, when a schema defines one.

schema = JSI.new_schema({"$schema": "http://json-schema.org/draft-07/schema",
  "additionalProperties": {"default": {"foo": true}}
})
jsi = schema.new_jsi({})
jsi['?']
#=> #{<JSI> "foo" => true}

The meta-schemas use default values for schemas and other things, in particular defaults of an empty schema (draft 6) or true schema (draft 7) where a schema is described.

# schema with no keywords
schema = JSI.new_schema({"$schema": "http://json-schema.org/draft-06/schema"})
# any subschema is an empty schema
schema.additionalProperties
#=> #{<JSI (JSI::JSONSchemaOrgDraft06) Schema>}
schema.properties['?']
#=> #{<JSI (JSI::JSONSchemaOrgDraft06) Schema>}

# and recursively
schema.additionalProperties.additionalProperties.additionalProperties.additionalProperties
#=> #{<JSI (JSI::JSONSchemaOrgDraft06) Schema>}

This behavior seems unintuitive and potentially problematic. Child schemas which are not present returning nil seems more what one would expect.
Specifications after draft 7 no longer specify a default value for a schema, but these are much less widely in use (and not supported in JSI as of this writing).

JSI could, by default, not return a default value when the default comes from a meta-schema. Having different rules for schemas than for other instances is not my favorite, but I already do similar to decide whether to instantiate a child as a JSI so that boolean schemas are returned as schemas instead of plain booleans.
Or JSI could always default to not return defaults values.

[notEthan]

Addressed in #310