#6101 Let users define objectId (#6177)

* #6101 Let users define objectId

* Add `allowCustomObjectId` to PS Option

* Add checking in objectId creation

* Add test

* Update docs

* Update definition

* Change default to false

* throw on empty, null, undefined

* better tests

* unused async

* removed comment

* retain comment

* Linting fix according to contributing spec.

src/RestWrite.js

@@ -46,17 +46,32 @@ function RestWrite(
   this.storage = {};
   this.runOptions = {};
   this.context = {};
-  if (!query && data.objectId) {
-    throw new Parse.Error(
-      Parse.Error.INVALID_KEY_NAME,
-      'objectId is an invalid field name.'
-    );
-  }
-  if (!query && data.id) {
-    throw new Parse.Error(
-      Parse.Error.INVALID_KEY_NAME,
-      'id is an invalid field name.'
-    );
+
+  if (!query) {
+    if (this.config.allowCustomObjectId) {
+      if (
+        Object.prototype.hasOwnProperty.call(data, 'objectId') &&
+        !data.objectId
+      ) {
+        throw new Parse.Error(
+          Parse.Error.MISSING_OBJECT_ID,
+          'objectId must not be empty, null or undefined'
+        );
+      }
+    } else {
+      if (data.objectId) {
+        throw new Parse.Error(
+          Parse.Error.INVALID_KEY_NAME,
+          'objectId is an invalid field name.'
+        );
+      }
+      if (data.id) {
+        throw new Parse.Error(
+          Parse.Error.INVALID_KEY_NAME,
+          'id is an invalid field name.'
+        );
+      }
+    }
   }
 
   // When the operation is complete, this.response may have several