Server header not disabled on Jetty when SSL is used

[ksperling]

With #4730 the Server header is meant to be disabled by default, but this only works when SSL is not used. Given that disabling the server header is primarily for security reasons, having it work with SSL is pretty essential.

This is because the HTTPS and HTTP connector factories in JettyEmbeddedServletContainerFactory don't configure the HttpConfiguration in the same way.

Tested with Spring Boot 1.4.2.RELEASE with the default Jetty version (9.3.14)