[aws|rds] Server#destroy argument is optional

By default Server#destroy does not take a final
snapshot.

This makes the Server#destroy API more consistent
with other models.

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

@@ -45,7 +45,7 @@ module Fog
           state == 'available'
         end
    
-        def destroy(snapshot_identifier)
+        def destroy(snapshot_identifier=nil)
           requires :id
           connection.delete_db_instance(id, snapshot_identifier, snapshot_identifier.nil?)
           true
@@ -102,4 +102,4 @@ module Fog
       end
     end
   end
-end
+end