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 <[email protected]> Reviewed-by: Nick Cooper <[email protected]> Reviewed-by: Nigel Tao <[email protected]>