runtime: polyfill Object.values()

Minimum supported Node version is 6.9.0, but Object.values() was introduced in
Node < 7. Let's use a polyfill if needed.

This will be removed when minimum supported Node version is raised to 8.9.0.

Fixes #3459

src/node/stats.js

@@ -1,3 +1,19 @@
+/*
+ * TODO: this polyfill is needed for Node 6.9 support.
+ *
+ * Once minimum supported Node version is raised to 8.9.0, it will be removed.
+ */
+if (!Object.values) {
+  var log4js = require('log4js');
+  var statsLogger = log4js.getLogger("stats");
+
+  statsLogger.warn(`Enabling a polyfill to run on this Node version (${process.version}). Next Etherpad version will remove support for Node version < 8.9.0. Please update your runtime.`);
+
+  var values = require('object.values');
+
+  values.shim();
+}
+
 var measured = require('measured-core')
 
 module.exports = measured.createCollection();

src/package.json

@@ -45,6 +45,7 @@
     "log4js": "0.6.35",
     "measured-core": "1.11.2",
     "npm": ">=4.0.2",
+    "object.values": "^1.0.4",
     "request": "2.83.0",
     "resolve": "1.1.7",
     "security": "1.0.0",