#259 Open isUsernameValid for subclassing support

[Peter Rossbach]

Hello,

at the centaurus-platform project (centaurus.sf.net) I have
patch the HttpAdaptor.java (2.1.1) to integrated Tomcat
Realms as auth data source.

Can you please open the isUserNameValid method for
better subclassing support?

L696 HttpAdaptor
protected boolean isUsernameValid(String username,
String password)

instead
private boolean ...

Thanks
Peter