sync CHANGES for 0.18.3
sync CHANGES for 0.18.3

marklrh/ocaml-cohttp,edwintorok/ocaml-cohttp,mor1/ocaml-cohttp,mor1/ocaml-cohttp,marklrh/ocaml-cohttp,inhabitedtype/ocaml-cohttp,Drup/ocaml-cohttp,inhabitedtype/ocaml-cohttp,marklrh/ocaml-cohttp,inhabitedtype/ocaml-cohttp,Drup/ocaml-cohttp,edwintorok/ocaml-cohttp,edwintorok/ocaml-cohttp,mor1/ocaml-cohttp,Drup/ocaml-cohttp