add dummy implementation for mitmConn.LocalAddr()

Fixes panic due to unimplemented method when the stdlib http server
calls this under certain conditions.
For our mitm implementation of net.Conn, an empty address string should be sufficient.

proxy/mitmconn.go

@@ -48,7 +48,7 @@ func (c *mitmConn) RemoteAddr() net.Addr {
 }
 
 func (c *mitmConn) LocalAddr() net.Addr {
-	panic("not implemented")
+	return addr("")
 }
 
 func (c *mitmConn) SetDeadline(t time.Time) error {