Fix bugs in http-client discovered by Piotr Klibert

[cgay]

• $default-http[s]-port shouldn't be the same for client and server, so I removed it from http-common and instead use unexported constants in server and client. Client gets 80 and 443, server gets 8000 and 8443. Eventually the client shouldn't need these constants since the uri library should supply defaults for schemes that specify them.
• Default read-response-body? to #t in http-request(, ).
• Use parse-url instead of parse-uri in http-request because expects a . This may not be the correct long-term fix though. Will look into getting rid of the class completely and just have .