[DOC] proto void header_remove([string header_name])
      Removes an HTTP header previously set using header()
      The header_name parameter is optionnal, all headers are
      removed if it is not set

[SAPIs] The header_handler callback in sapi_module_struct has
           been changed, it now take a new argument.

           When it is set to SAPI_HEADER_DELETE, sapi_header->header is
           the name of an header, header_handler has to delete it.

           When it is set to SAPI_HEADER_DELETE_ALL, header_handler has
           to delete all headers.

           When sapi_header_op_enum is SAPI_HEADER_ADD or _REPLACE,
           sapi_header->header is in the form "Name: value", header_handler
           has to add or replace the given header.

           In all cases, header_handler must not free sapi_header or
           sapi_header->header. SAPI_HEADER_ADD must be returned if the
           header has been added or replaced, or 0 in other cases.