turn mitm conn Set[Read|Write]Deadline methods into noops

Timeouts should be handled by the http stack for the underlying tcp
connections so noops are appropriate for this virtual connection. No
need to panic.

proxy/mitmconn.go

@@ -52,15 +52,15 @@ func (c *mitmConn) LocalAddr() net.Addr {
 }
 
 func (c *mitmConn) SetDeadline(t time.Time) error {
-	panic("not implemented")
+	return nil
 }
 
 func (c *mitmConn) SetReadDeadline(t time.Time) error {
-	panic("not implemented")
+	return nil
 }
 
 func (c *mitmConn) SetWriteDeadline(t time.Time) error {
-	panic("not implemented")
+	return nil
 }
 
 type addr string