Don't barf on POST or PUT with complex "Content-Type:"

SNOOZE-COMMON:PARSE-CONTENT-TYPE-HEADER presently ignores parameters
in the "Content-Type:" header value, such as "Charset=UTF-8", but at
least it sees the basic content type descriptor, whereas it previously
returned nil.

* common.lisp (content-classes-in-accept-string): Use
PARSE-CONTENT-TYPE-HEADER.
(parse-content-type-header): Use RFC2388:PARSE-HEADER.

* snooze.asd (:snooze): Use RFC2388 system.