Request.authenticate(Response) should return true if already authenticated

[janbartel]

Jetty 10

https://github.com/eclipse-ee4j/jakartaee-tck/blob/master/src/com/sun/ts/tests/common/jspservletsec/secformClient.java#L1731

This test performs a programmatic login (Request.login()), then a programmatic authentication (Request.authenticate()).

According to the javadoc, Request.authenticate() should return:

• true if all of the following are not null: request.getUserPrincipal(), request.getRemoteUser(), request.getAuthType()
• false if the authentication mechanism has sent a challenge (ie an authentication requiring multiple round-trips to the client is in progress)
• throw a ServletException if the authentication mechanism failed the authentication but hasn't already called sendError 401

Our Request.authenticate() method currently does a sendError 401 if the user has already been authenticated.