Proxies, Request-URI, and digest-uri

Is it optional, or required, for proxy servers to rewrite the Request-URI
from absoluteURI to abs_path form? I can't find where it is specified that
it is allowed, except this reference in 2617: (3.2.2)

   digest-uri
     The URI from Request-URI of the Request-Line; duplicated here
     because proxies are allowed to change the Request-Line in transit.

If it *is* up to the proxy server whether it rewrites the Request-URI or
not, I think that makes 2617 sec 3.2.2.5 an impossible demand? Regarding
the value of the digest-uri field submitted in an Authorization: header.

                                           In particular, it MUST
   be an "absoluteURL" if the Request-URI is an "absoluteURL".

... if the proxy may change Request-URI from absoluteURI to abs_path, or
it may leave it as an absoluteURI, how will the client know which to
submit in digest-uri?

(a typo, too: that section uses "absoluteURL" twice... should be
absoluteURI?)

Regards,

joe