spec: fix logic error in test

this test was checking for the wrong value using the
wrong function. and that wrong function changed its
response type in a breaking fashion between jquery
1.7 and jquery 1.8.

test claimed to be checking if due date element was
disabled, but it was actually checking if the due
date element was *not* disabled.  Also the due date
enable/disable code path wasn't even exercised by
the test, which is why the incorrectly negated check
was passing on jquery 1.7

flag = none

Test Plan: tests pass

refs FOO-4220

Change-Id: I878f7514e96ee6cc62ba0c3f7b0f95550cb70d25
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/338205
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Charley Kline <ckline@instructure.com>
Product-Review: Charley Kline <ckline@instructure.com>
QA-Review: Charley Kline <ckline@instructure.com>

spec/coffeescripts/calendar/EditAssignmentDetailsSpec.js

@@ -258,5 +258,6 @@ test('Should disable changing the date if course pacing is enabled', function ()
   this.event.contextInfo = {course_pacing_enabled: true}
   const view = createView(commonEvent(), this.event)
   view.setContext('course_3')
-  equal(view.$('#assignment_due_at').css('disabled'), '')
+  view.contextChange({target: '#assignment_context'}, false)
+  equal(view.$('#assignment_due_at').prop('disabled'), true)
 })