Add wrq:req_headers_to_list to expose headers in a normalized form

This avoids the need for something like
mochiweb_headers:to_list(wrq:req_headers(RD)), which breaks the backend
abstration.
Merge pull request #5 from Vagabond/adt-req_headers_to_list

Add wrq:req_headers_to_list to expose headers in a normalized form