PlaceholderBlock should implement RenderCommand

If we have an empty zone like this:
<t:zone t:id="myZone" />

zone.getBody() fails to update the zone in an event handler with the following exception:

org.apache.tapestry5.ioc.util.UnknownValueException
A component event handler method returned the value <PlaceholderBlock>. Return type org.apache.tapestry5.internal.structure.ComponentPageElementImpl$PlaceholderBlock can not be handled.

This is because ComponentPageElementImpl.PlaceholderBlock does not implement RenderCommand.

A patch is provided with a test that fails.

I would highly appreciate it if a commiter applies it to the 5.4. branch and to the 5.3. branch.

Cheers,
Dragan Sahpaski