Proxy deserialization fails due to classloading issues

JavaSerializer.ClassResolverObjectInputStream overrides resolveClass to resolve classes via the Wicket ClassResolvers. This does however not happen for resolveProxyClass.

An example of how this can go wrong:

• A page with a large component tree is deserialized (war).
• LinkedMap (used in MarkupContainer) is loaded in a parent ClassLoader (ear).
• Via this stack, a proxy is hit implementing Spring classes (from the war)
• Due to LinkedMap determining the latestUserDefinedLoader, the ear-loader is used for the lookup of this interface, which fails

Unfortunately, writing a testcase for this is not easy, so I only have a proposed fix: see the classloadingfix branch