The call to an unsecure registry doesn't return an error saying that the
"server gave an HTTP response to an HTTPS client" but a
tls.RecordHeaderError saying that the "first record does not look like a
TLS handshake", this changeset looks for the right error for that case.
This fixes the http fallback when using an insecure registry
Signed-off-by: Djordje Lukic <djordje.lukic@docker.com>