1. Changing an existing header type:
- modify the Type of the header in Headers.hxx
class Expires_Header : public HeaderBase
{
   public:
      typedef ExpiresCategory Type; // this defines the type
      virtual Headers::Type getTypeNum() const {return Headers::Expires;}
      Expires_Header()
      {
         Headers::CommaTokenizing[Headers::Expires] = Type::isCommaTokenizing;
         Headers::HeaderNames[Headers::Expires] = Symbols::Expires;
      }
};
- define the new derived ParserCategory according to the ParserCategoy interface

2. Adding a new header.
- determine its type.
- add to HeaderTypes.hxx Type enum.
- create a type class in Headers.hxx
- update the gperf functions in Headers.cxx
  - change the hash and in_word_set function names
  - make the comparisons case insensitive
  - use strcasencmp -- Data not NULL terminated