Fix bugs in http-client discovered by Piotr Klibert

* $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(<string>, <request-method>).

* Use parse-url instead of parse-uri in http-request because
  <base-http-request> expects a <url>.  This may not be the correct long-term
  fix though.  Will look into getting rid of the <url> class completely and
  just have <uri>.