Making Server response header consistent.

noticed while testing, that static resources
sometime respond with Nexus as Server, and sometimes
as Jetty.

It turned out, that only GETs would have proper
Server header set. Applying same solution as present
in ContentServlet.

Ultimately, this should be set by some filter, and not
have to fiddle with these "standard" headers (common
to all responses from server) on per-servlet basis.
Fixing ugly NPE/log pollition

If someone (by browser or curl) accesses the ErrorPageServlet.
As this request would be "unexpected", an NPE happened in it,
that caused nasty stack trace in log, and then ErrorFilter
would kick in, spitting the stack trace again (double NPE
in logs).

It was simply matter of Integer/int autoboxing and casting.
Now a simple check is added, to detect when page is
accessed externally and providing somewhat sensible
404 not found response instead of 500 Server Error
(and stack trace in log).