Add tests for large headers, both individual fields and the number
Add tests for large headers, both individual fields and the number

This is to check for stack overflows in #398, but none are showing
up with these particular checks

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