re-added nickname construction for RWADef > Swagger conversion (https://github.com/restlet/apispark/issues/1328)

the nickname is computed from HTTP method name and resource path. It is in fact: method to lower case + resourcepath to camel case (removed slashes and {})
moved nickname modification to APISpark extension (https://github.com/restlet/apispark/issues/1328)
removed items object from Swagger file when it is empty in both Swwagger and AS extensions (https://github.com/restlet/apispark/issues/1328)