Correct expected status code for deregistration

Per the Eureka documentation and the implementation in method
com.netflix.eureka.resources.InstanceResource#cancelLease(), Eureka
returns HTTP status code 200 to indicate successful deregistration of an
instance, and not status code 204 as our EurekaConnection's
DeregisterInstance method had expected.

net.go

@@ -217,7 +217,9 @@ func (e *EurekaConnection) DeregisterInstance(ins *Instance) error {
 		log.Errorf("Could not complete deregistration, error: %s", err.Error())
 		return err
 	}
-	if rcode != 204 {
+	// Eureka promises to return HTTP status code upon deregistration success, but fargo used to accept status code 204
+	// here instead. Accommodate both for backward compatibility with any fake or proxy Eureka stand-ins.
+	if rcode != 200 && rcode != 204 {
 		log.Warningf("HTTP returned %d deregistering Instance=%s App=%s", rcode, ins.Id(), ins.App)
 		return fmt.Errorf("http returned %d possible failure deregistering instance\n", rcode)
 	}