Corrected rounding function when letter grade is computed

A rounding error on 10 pt scale was causing 0.87 to display as a B instead of B+

fixes CNVS-11843

test plan:
- Create assignment worth 10 total points and score it by letter grade
- Use default grading scheme so B+ is >90% - 87%
- Save the assignment and as the test student submit to it
- In gradebook score it with an 8.7/10, should show B+ as grade
- Update score to 8.69/10, should show B as grade

Change-Id: I1c8abbaa53c49eb019cb19798ddc232bf65f9056
Reviewed-on: https://gerrit.instructure.com/38759
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>

app/models/assignment.rb

@@ -18,6 +18,7 @@
 
 require 'set'
 require 'canvas/draft_state_validations'
+require 'bigdecimal'
 
 class Assignment < ActiveRecord::Base
   include Workflow
@@ -680,7 +681,7 @@ class Assignment < ActiveRecord::Base
       result = passed ? "complete" : "incomplete"
     when "letter_grade", "gpa_scale"
       if self.points_possible.to_f > 0.0
-        score = score.to_f / self.points_possible.to_f
+        score = (BigDecimal.new(score.to_s) / BigDecimal.new(points_possible.to_s)).to_f
         result = grading_standard_or_default.score_to_grade(score * 100)
       elsif given_grade
         # the score for a zero-point letter_grade assignment could be considered

spec/models/assignment_spec.rb

@@ -474,6 +474,22 @@ describe Assignment do
       @submission.user_id.should eql(@user.id)
     end
 
+    it "should properly calculate letter grades" do
+      @assignment.grading_type = 'letter_grade'
+      @assignment.points_possible = 10
+      @assignment.save!
+      grade = @assignment.score_to_grade(8.7)
+      grade.should eql("B+")
+    end
+
+    it "should properly allow decimal points in grading" do
+      @assignment.grading_type = 'letter_grade'
+      @assignment.points_possible = 10
+      @assignment.save!
+      grade = @assignment.score_to_grade(8.6999)
+      grade.should eql("B")
+    end
+
     it "should preserve letter grades grades with nil points possible" do
       @assignment.grading_type = 'letter_grade'
       @assignment.points_possible = nil