More Patches + Two New Filters

[Paul Curtis <pc...@terrapin.com>]

A couple of more patches for the Request & CocoonServletRequest to allow
some request information to get through to filters. I've also added two
filters:

  SessionFilter: Allows access to the servlet session object.
  RequestHeadersFilter: Allows access to the request headers.

The request headers filter does not return all the headers. I've only
included the ones enumerated by the Request object. The session filter
returns the values stored in the session object as strings.

Sample .xconf:
  <filter     name="headers"  
class="org.apache.cocoon.filters.RequestHeadersFilter"/>
  <filter     name="session"  
class="org.apache.cocoon.filters.SessionFilter"/>

Sample .xml:
   <request-headers
xmlns="http://xml.apache.org/cocoon/RequestHeaders"/>
   <servlet-session xmlns="http://xml.apache.org/cocoon/Session"/>