Previously, if the following happened:
* New Session Created
* Exception thrown
* Exception processed by error handler within Servlet
* Error Handler used a session
The result would be two sessions were created. This means the
data from the first session was also lost. This happend
because ERROR dispatch is a separate Filter invocation where
the request is no longer wrapped.
This commit ensures that currentSession is saved on a
HttpServletRequest attribute so that the ERROR dispatch sees
that a session was already created.
Fixes: gh-229
3d93f2cf56