Fix for refreshing schema after update

[rafael]

Description

I think there was a bug introduced in 3539 where after updating a vschema, vtgate is not properly updating it's version of it.

Steps to reproduce

1. Add a new table schema to a keyspace with:

vtctlclient ApplyVSchema -vschema_file schema-file keyspace

2. Updated schema is not shown in /debug/vschema and vtgate can't issue queries to that table.

After some debugging, I noticed that the callback was always passing nil in the changes and the vschema doesn't get updated.

This PR fixes this and seems to be working as expected. However, not sure if I'm missing something and nil here is needed.

[derekperkins]

This is likely the cause of #3567

[alainjobart]

Ouch I'm sorry about that...

Looking at the test, we have a test case for this, except the test is broken: can you also change resilient_server_test.go line 239 from 'ks1' to 'ks2', so the value we try to update is actually different? With that test change, it fails, and with your fix, it passes as it should... Thanks!