Update Firestore.runTransaction to resolve promise with passed function's value

See https://cloud.google.com/nodejs/docs/reference/firestore/0.14.x/Firestore#runTransaction

> If the transaction completed successfully or was explicitly aborted (by the updateFunction returning a failed Promise), the Promise returned by the updateFunction will be returned here

Author: squirly

src/firestore.js

@@ -63,8 +63,13 @@ MockFirestore.prototype.runTransaction = function(transFunc) {
     return doc.get();
   };
   return new Promise(function(resolve, reject) {
-    transFunc(batch).then(function() {
-      batch.commit().then(resolve).catch(reject);
+    Promise.resolve(transFunc(batch)).then(function(value) {
+      batch
+        .commit()
+        .then(function () {
+          resolve(value);
+        })
+        .catch(reject);
     }).catch(reject);
   });
 };

test/unit/firestore.js

@@ -125,10 +125,12 @@ describe('MockFirestore', function () {
             transaction.update(db.doc('doc'), {
               name: 'abc'
             });
+            return 'cba';
           });
-        }).then(function() {
+        }).then(function(transactionReturn) {
           db.doc('doc').get().then(function(doc2) {
             expect(doc2.get('name')).to.equal('abc');
+            expect(transactionReturn).to.equal('cba');
             done();
           }).catch(done);
         }).catch(done);