fix updating of *just* sis id via logins api

fixes gh-492

test plan:
 * change a user's sis id (but *just* their sis id) via the api
 * it should succeed

Change-Id: I2d35632a71895f980cb1a0d23a8a472e95275ca1
Reviewed-on: https://gerrit.instructure.com/38850
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>