reset x-forwarded-for before next req;

assume the following stack: cpp <- rproxyA <- rproxyB <- WAN

if A also accepts WAN requests, and A muxes both B and WAN
onto a single connection to cpp, then WAN requests may get
tagged with the IP-address of the most recent B request

aside from the confusing logs, this could break
unpost on servers with shared accounts

copyparty/httpconn.py

@@ -224,3 +224,6 @@ class HttpConn(object):
             if self.u2idx:
                 self.hsrv.put_u2idx(str(self.addr), self.u2idx)
                 self.u2idx = None
+
+            if self.rproxy:
+                self.set_rproxy()