lfs: debug print HTTP headers and JSON payload received from the server This has been extremely valuable to show divergences between `hg serve` and `lfs-test-server`. Once the `hg serve` code lands, there will be a certain amount of conditionalizing that needs to be done, because `lfs-test-server` doesn't always follow its spec. The $ISO_8601_DATE_TIME$ pattern masks the fact that `lfs-test-serve` is sending out an expires_at value of "0001-01-01T00:00:00Z". `hg serve` will (probably) use current time + 10 minutes or similar. The $HTTP_DATE$ is the current time.