Fix infinite recursion on ref

Previously, when a document have a reference to another document, we
ended up in an infinite loop. This is now prevented by only follow plain
objects.

This logic makes it easier to reason about the call chain too.

Author: op

src/utils.js

@@ -128,12 +128,9 @@ exports.removeEmptyRtdbProperties = function removeEmptyRtdbProperties(obj) {
 };
 
 exports.removeEmptyFirestoreProperties = function removeEmptyFirestoreProperties(obj) {
-  var t = typeof obj;
-  if (t === 'boolean' || t === 'string' || t === 'number' || t === 'undefined') {
+  if (!_.isPlainObject(obj)) {
     return obj;
   }
-  if (obj instanceof Date) return obj;
-  if (obj instanceof Timestamp) return obj;
 
   var keys = getKeys(obj);
   if (keys.length > 0) {

test/unit/firestore-document.js

@@ -199,6 +199,21 @@ describe('MockFirestoreDocument', function () {
 
       db.flush();
     });
+
+    it.only('sets value of doc with ref', function (done) {
+      var ref = db.doc('ref');
+      ref.create();
+      doc.set({
+        ref: ref
+      });
+      doc.get().then(function(snap) {
+        expect(snap.exists).to.equal(true);
+        expect(snap.get('ref')).to.have.property('ref');
+        done();
+      }).catch(done);
+
+      db.flush();
+    });
   });
 
   describe('#set with {merge: true}', function () {