spec: fix merge proxy spec on mysql

mysql2 adapter returns integers as integers, unlike postgres (in
rails 2). so just use a string as our constant, instead of an
integer

refs CNVS-7032

Change-Id: I79f6e786e677e8c6b69ddaeef93433358abb8376
Reviewed-on: https://gerrit.instructure.com/28623
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>