Hair up handling of HTTP headers. In new model, there are codecs for

Hair up handling of HTTP headers.  In new model, there are codecs for
each known header (basically almost all of RFC 2616 at this point).
These codecs translate between the string representation of a header
value and its internal representation.  MAKE-HTTP-HEADER accepts
either the string representation or the internal representation.
HTTP-HEADER-VALUE always contains the string representation, while
HTTP-HEADER-PARSED-VALUE contains the internal representation.  If the
decoder for a header fails on a particular string represenation, or if
there's no decoder for that header, HTTP-HEADER-PARSED-VALUE contains
a default object (use DEFAULT-OBJECT? to test for it).

Additionally, HTTP requests have been changed so that the METHOD is a
string rather than a symbol (that is, "GET" rather than '|GET|).