fix complete/incomplete assignment editing bug

fixes CNVS-1501

Test plan:
  * make a complete/incomplete assignment
  * mark some students complete, some students incomplete
  * change the number of points possible on the assignment
  * the complete/incomplete status of each submission should be retained

Change-Id: Ice61d884936c908de977e1f2abdf99cbb26210ae
Reviewed-on: https://gerrit.instructure.com/36377
Reviewed-by: Liz Abinante <labinante@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>

app/models/assignment.rb

@@ -636,15 +636,13 @@ class Assignment < ActiveRecord::Base
     when "percent"
       result = "#{score_to_grade_percent(score)}%"
     when "pass_fail"
-      if self.points_possible.to_f > 0.0
-        passed = score.to_f == self.points_possible.to_f
+      if points_possible && points_possible > 0
+        passed = score > 0
       elsif given_grade
         # the score for a zero-point pass/fail assignment could be considered
         # either pass *or* fail, so look at what the current given grade is
         # instead
         passed = ["complete", "pass"].include?(given_grade)
-      else
-        passed = score.to_f > 0.0
       end
       result = passed ? "complete" : "incomplete"
     when "letter_grade", "gpa_scale"

spec/models/assignment_spec.rb

@@ -2450,6 +2450,18 @@ describe Assignment do
       @assignment.save!
       s.reload.version_number.should == 2
     end
+
+    it "works for pass/fail assignments" do
+      student1, student2 = n_students_in_course(2)
+      a = @course.assignments.create! grading_type: "pass_fail", points_possible: 5
+      sub1 = a.grade_student(student1, grade: "complete").first
+      sub2 = a.grade_student(student2, grade: "incomplete").first
+
+      a.update_attribute :points_possible, 10
+
+      sub1.reload.grade.should == "complete"
+      sub2.reload.grade.should == "incomplete"
+    end
   end
 
   describe '#graded_count' do