'entity_class' Configuration Property

Currently `zf-apigility`'s `module.config.php` makes mention of an available `entity_class` attribute, but as far as I can tell `zf-rest`'s is the only configuration ever used for the `entity_class` property (across all included packages). This patch updates the configuration retrieval to default the `zf-rest` configuration to it's own (`zf-apigility => db-connected`) rather than an empty array, allowing for expected behavior with either configuration setting.

Also, if the `entity_class` property itself is just an artifact from early development, feel free to ignore!