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.

cc: @paul.erickson @levett @bhill6