fix: correct handling of HttpServletRequest#getHeaders and tests
This pull request addresses issue #6.
Properly handles cases where container may return blank String values in the returned list. Tests were previously adding List objects as headers - need to add just String values.
See the javadocs for HttpServletRequest#getHeaders for reference.
I intend to release 1.6.1 when this is merged.
Edit: this also corrects the case of the name of the Manifest isMemberOf
header.