merge promotion results

* will update the state to "modifying"

lib/fog/aws/models/rds/server.rb

@@ -88,7 +88,9 @@ module Fog
         def promote_read_replica
           requires :id
 
-          service.promote_read_replica(id)
+          data = service.promote_read_replica(id).body["PromoteReadReplicaResult"]["DBInstance"]
+
+          merge_attributes(data)
         end
 
         alias promote promote_read_replica

tests/models/rds/server_tests.rb

@@ -102,8 +102,7 @@ Shindo.tests("AWS::RDS | server", ['aws', 'rds']) do
     end
 
     tests('#promote_read_replica').succeeds do
-      replica.promote
-      replica.wait_for { state != "modifying" }
+      replica.promote.wait_for { state != "modifying" }
 
       replica.read_replica_source == nil
     end