Flatten threads: fix for application/json+protobuf responses

When a new message is created in a thread, the thread itself is
reloaded, but via an application/json+protobuf request (array-like data)
instead of a regular text/plain request (object-like data). Since the
code didn't work well for these types of requests, the thread didn't
fully load.

This CL fixes this issue by correctly handling application/json+protobuf
responses in the response modifiers.

An issue with the read-only interceptors has also been fixed, and tests
have been added to ensure that the array-like to object-like and
viceversa transformation functions work properly.

Bug: twpowertools:153
Change-Id: If6cd5adc67d676bf36986f325e791124fa71da51
diff --git a/.eslintrc.json b/.eslintrc.json
index 185c15a..6d1c0a4 100644
--- a/.eslintrc.json
+++ b/.eslintrc.json
@@ -6,6 +6,15 @@
     "extends": "eslint:recommended",
     "overrides": [
+      {
+        "files": [
+          "**/*.test.js",
+          "**/*.test.mjs"
+        ],
+        "env": {
+          "jest": true
+        }
+      }
     "parserOptions": {
         "ecmaVersion": "latest",