added user_image to basic_lti closes #CNVS-1310

test plan:
1. run the specs

Change-Id: Ie2ca5ba3c12eac62818570f19ba82825a2cadfd1
Reviewed-on: https://gerrit.instructure.com/15893
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Joe Tanner <joe@instructure.com>
QA-Review: Ryan Florence <ryanf@instructure.com>

lib/basic_lti.rb

@@ -83,6 +83,7 @@ module BasicLTI
       hash['resource_link_id'] = link_code
       hash['resource_link_title'] = tool.name
       hash['user_id'] = user.opaque_identifier(:asset_string)
+      hash['user_image'] = user.avatar_url
       hash['roles'] = user.lti_role_types(context).join(',') # AccountAdmin, Student, Faculty or Observer
       if tool.include_name?
         hash['lis_person_name_given'] = user.first_name

spec/lib/basic_lti_spec.rb

@@ -115,6 +115,7 @@ describe BasicLTI do
       hash['resource_link_id'].should == '123456'
       hash['resource_link_title'].should == @tool.name
       hash['user_id'].should == @user.opaque_identifier(:asset_string)
+      hash['user_image'].should == @user.avatar_url
       hash['roles'].should == 'Instructor'
       hash['context_id'].should == @course.opaque_identifier(:asset_string)
       hash['context_title'].should == @course.name