*Motivation*

The getSchema logic in Pulsar proxy handler doesn't handle the case that
the request doesn't have schema version.

*Modification*

- Fix the logic to handle the case that a GetSchema request doesn't have schema version.
- Forward the GetSchemaResponse back to the client

*Tests*

The GetSchema tests in ProxyTest was disabled by mistake. Turn it on.
(cherry picked from commit 676fd71a)