do not create menelaus-auth-token header if token is undefined
do not create menelaus-auth-token header if token is undefined

... so menelaus_auth:get_token/1 returns undefined, not "undefined"

Change-Id: I106853123b63067a5df9d5464f27c7060d811bb0
Reviewed-on: http://review.couchbase.org/71952
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>
Tested-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>

membase/ns_server,membase/ns_server,membase/ns_server,ceejatec/ns_server,membase/ns_server,membase/ns_server,ceejatec/ns_server,ceejatec/ns_server,ceejatec/ns_server,ceejatec/ns_server,membase/ns_server,ceejatec/ns_server