record grader when grading via API

test plan:
 * go to gradebook2 and change a grade
 * go to /gradebook/history for the course, and view the grade change
   you just made. it should say it was you that did it

Change-Id: Ie3f91b7ceadbe99b3cccff33a957595e5d8c2cf6
Reviewed-on: https://gerrit.instructure.com/13653
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>

app/controllers/submissions_api_controller.rb

@@ -259,7 +259,7 @@ class SubmissionsApiController < ApplicationController
     end
 
     if authorized
-      submission = {}
+      submission = { :grader => @current_user }
       if params[:submission].is_a?(Hash)
         submission[:grade] = params[:submission].delete(:posted_grade)
       end

spec/apis/v1/submissions_api_spec.rb

@@ -136,6 +136,7 @@ describe 'Submissions API', :type => :integration do
 
       Submission.count.should == 2
       @submission = Submission.last(:order => :id)
+      @submission.grader.should == @teacher
 
       json['score'].should == 7.5
       json['grade'].should == '75%'