Upgrade Wicket to latest stable release - 6.12
Disable SSL connector in jetty-maven-plugin

Since there is no 'keystore' file in the resources of this app "mvn jetty:run" cannot be used to start it.
Do not buffer the response.

This way Wicket is more close to the behavior of the other web framework
Non-functional changes.

Formatting and comments
Correct the version of Wicket (6.12.0)
Issue #639 - Write back single object if there is just one entity requested
Merge pull request #639 from martin-g/master

Do not cache the response for Wicket test application