prefer array for available_canvas_resources variable

test plan:
* launching a tool with the
 module_index_menu and assignment_index_menu placements
 should return an array of hashes for the
 com_instructure_course_available_canvas_resources
 variable, as described in the documentation

refs #LA-75

Change-Id: I2c81ca611a60dfdc67d62823b35319353c4d846d
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/218693
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Tested-by: Jenkins
Reviewed-by: Carl Kibler <ckibler@instructure.com>
QA-Review: Carl Kibler <ckibler@instructure.com>
Product-Review: James Williams <jamesw@instructure.com>

lib/lti/variable_expander.rb

@@ -1355,7 +1355,11 @@ module Lti
     #   [{"id":"3","name":"First Module"},{"id":"5","name":"Second Module"}]
     #   ```
     register_expansion 'com.instructure.Course.available_canvas_resources', [],
-                       -> { @request.parameters['com_instructure_course_available_canvas_resources']&.to_json },
+                       -> {
+                         val = @request.parameters['com_instructure_course_available_canvas_resources']
+                         val = val.values if val.is_a?(Hash)
+                         val&.to_json
+                       },
                        default_name: 'com_instructure_course_available_canvas_resources'
 
     private