Added serializer for session.all

refs: https://github.com/TryGhost/Toolbox/issues/245

- Session didn't have a serializer because it's deliberately a passthrough
- With the upcoming refactor we want to have all the serializers defined explicitly
- This will allow us to change the default behaviour
- Tests were added to cover this endpoint in 2cf7e00493

core/server/api/canary/utils/serializers/output/index.js

@@ -159,5 +159,9 @@ module.exports = {
 
     get slack() {
         return require('./slack');
+    },
+
+    get session() {
+        return require('./session');
     }
 };

core/server/api/canary/utils/serializers/output/session.js

@@ -0,0 +1,9 @@
+const debug = require('@tryghost/debug')('api:canary:utils:serializers:output:session');
+
+module.exports = {
+    all(data, apiConfig, frame) {
+        debug('all');
+
+        frame.response = data;
+    }
+};