fix: throws when fetching object without id (#641)

Author: sdjdd

src/file.js

@@ -619,6 +619,9 @@ module.exports = function(AV) {
        *     completes.
        */
       fetch(fetchOptions, options) {
+        if (!this.id) {
+          throw new Error('Cannot fetch unsaved file');
+        }
         var request = AVRequest(
           'files',
           null,

src/object.js

@@ -946,6 +946,9 @@ module.exports = function(AV) {
        *     completes.
        */
       fetch: function(fetchOptions = {}, options) {
+        if (!this.id) {
+          throw new Error('Cannot fetch unsaved object');
+        }
         var self = this;
         var request = _request(
           'classes',

test/file.js

@@ -188,6 +188,10 @@ describe('File', function() {
           .fetch()
           .then(file => (this.file = file));
       });
+      it('should throws when objectId is empty', () => {
+        const file = new AV.File('filename', { base64: 'dGVzdA==' });
+        expect(file.fetch).throwError();
+      });
       it('should retrieve all data', function() {
         var file = this.file;
         expect(file).to.be.a(AV.File);

test/object.js

@@ -447,6 +447,10 @@ describe('Objects', function() {
           expect(score.createdAt).to.be.a(Date);
           expect(score.id).to.be.eql(gameScore.id);
         }));
+    it('should throws when objectId is empty', () => {
+      const object = new AV.Object('GameScore');
+      expect(object.fetch).throwError();
+    });
     it('fetch should remove deleted keys', () => {
       const getFakedScore = () =>
         AV.parseJSON(