fix sis courses export report on a sub account

fixes CNVS-4439 FRD

test plan:
 * in a sub account, run the sis export report, selecting the courses
   option
 * it should work

Change-Id: I1fe42aa4ceeb59742376dfdbeeb7481152cf7286
Reviewed-on: https://gerrit.instructure.com/21598
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>

vendor/plugins/account_reports/lib/canvas/account_reports/sis_exporter.rb

@@ -193,7 +193,7 @@ module Canvas::AccountReports
 
         csv << headers
         courses = root_account.all_courses.includes(:account, :enrollment_term)
-        courses = courses.where("sis_source_id IS NOT NULL") if @sis_format
+        courses = courses.where("courses.sis_source_id IS NOT NULL") if @sis_format
 
         if @include_deleted
           courses = courses.where("courses.workflow_state<>'deleted' OR courses.sis_source_id IS NOT NULL")

vendor/plugins/account_reports/spec_canvas/sis_provisioning_reports_spec.rb

@@ -511,6 +511,13 @@ describe "Default Account Reports" do
                              nil,@default_term.id.to_s,nil,"concluded",nil,nil]
       end
 
+      it "should run the sis report report on a sub account" do
+        parameters = {}
+        parameters["courses"] = true
+        # all I care about is that it didn't throw a database error due to ambiguous columns
+        parsed = ReportSpecHelper.run_report(@sub_account,"sis_export_csv",parameters,3)
+      end
+
       it "should run the provisioning report report on a sub account" do
         parameters = {}
         parameters["courses"] = true