jetty-12 ee9 contextPath not set correctly on nested ContextHandler

[janbartel]

The deployer tries to set a default context path when it deploys a webapp. In the case of ee9, the webapp will be an instanced of the nested/ContextHandler, which is a Supplier<ContextHandler> ie a supplier of core ContextHandler. The deployer gets the core ContextHandler and calls setContextPath(String) on it. Unfortunately, it is only in the nested/ContextHandler.setContextPath(String) method (which is not called) where we set up the _contextPathEncoded. It is the _contextPathEncoded that the Request uses. Thus, it appears as if the webapp is deployed correctly at the default context path, but any code that uses Request.getContextPath() will return null. We see this with the tck form security tests, where the redirect to the /login.jsp page is missing the _contextPathEncoded prefix.

[janbartel]

Fixed via #9786