FELIX-4664 "Downgrade" Servlet API dependency to Servlet API 2.3 (the minimum required)
   and adapt the unit test with internal API to retrieve the header(s) for validation.

git-svn-id: https://svn.apache.org/repos/asf/felix/trunk@1629609 13f79535-47bb-0310-9956-ffa450edef68