webdav: log the X-Litmus header in the litmus_test_server.

This helps line up HTTP events (in the Go code) with the litmus test
suite (C code).

Change-Id: I2d0847069ae1304948ba1219b63cd438a4fa921c
Reviewed-on: https://go-review.googlesource.com/3760
Reviewed-by: Robert Stepanek <robert.stepanek@gmail.com>
Reviewed-by: Nick Cooper <nmvc@google.com>
Reviewed-by: Nigel Tao <nigeltao@golang.org>