encode available_canvas_resources variable as JSON

test plan:
* testing with the new index placement tools
 on the assignments/modules page should send
 the available canvas resources encoded
 as a JSON string so the oauth signature works

Change-Id: I329c819a17ca70b84a5d165964246af6e635dacb
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/218523
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

@@ -1346,16 +1346,16 @@ module Lti
                        -> { @request.parameters['com_instructure_course_allow_canvas_resource_selection'] },
                        default_name: 'com_instructure_course_allow_canvas_resource_selection'
 
-    # Returns a list of content groups which can be selected, providing ID and name of each group,
+    # Returns a JSON-encoded list of content groups which can be selected, providing ID and name of each group,
     # forwarded from the request.
     # Empty value if com.instructure.Course.allow_canvas_resource_selection is false.
     #
     # @example
     #   ```
-    #   [{"id": "3", name: "First Module"}, {"id": "5", name: "Second Module"]
+    #   [{"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'] },
+                       -> { @request.parameters['com_instructure_course_available_canvas_resources']&.to_json },
                        default_name: 'com_instructure_course_available_canvas_resources'
 
     private

spec/lib/lti/variable_expander_spec.rb

@@ -693,7 +693,7 @@ module Lti
       it 'has substitution for $com.instructure.Course.available_canvas_resources' do
         exp_hash = {test: '$com.instructure.Course.available_canvas_resources'}
         variable_expander.expand_variables!(exp_hash)
-        expect(exp_hash[:test]).to eq [{"id"=>"1", "name"=>"item 1"}, {"id"=>"2", "name"=>"item 2"}]
+        expect(JSON.parse(exp_hash[:test])).to eq [{"id"=>"1", "name"=>"item 1"}, {"id"=>"2", "name"=>"item 2"}]
       end
 
       context 'context is a group' do