compile gen_smtp session behavior first

To avoid erlang compiler warning

Change-Id: I707c065ef55421f9c664430d809afb261817c338
Reviewed-on: http://review.couchbase.org/35106
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>