fix export Bytes data type to JSON (#2409)

spec/MongoTransform.spec.js

@@ -156,6 +156,18 @@ describe('parseObjectToMongoObjectForCreate', () => {
     done();
   });
 
+  it('bytes', (done) => {
+    var input = {binaryData: "aGVsbG8gd29ybGQ="};
+    var output = transform.mongoObjectToParseObject(null, input, {
+      fields: { binaryData: { type: 'Bytes' }},
+    });
+    expect(typeof output.binaryData).toEqual('object');
+    expect(output.binaryData).toEqual(
+      {__type: 'Bytes', base64: "aGVsbG8gd29ybGQ="}
+    );
+    done();
+  });
+
   it('nested array', (done) => {
     var input = {arr: [{_testKey: 'testValue' }]};
     var output = transform.mongoObjectToParseObject(null, input, {

src/Adapters/Storage/Mongo/MongoTransform.js

@@ -781,6 +781,10 @@ const mongoObjectToParseObject = (className, mongoObject, schema) => {
             restObject[key] = GeoPointCoder.databaseToJSON(value);
             break;
           }
+          if (schema.fields[key] && schema.fields[key].type === 'Bytes' && BytesCoder.isValidDatabaseObject(value)) {
+            restObject[key] = BytesCoder.databaseToJSON(value);
+            break;
+          }
         }
         restObject[key] = nestedMongoObjectToNestedParseObject(mongoObject[key]);
       }
@@ -815,15 +819,29 @@ var DateCoder = {
 };
 
 var BytesCoder = {
+  base64Pattern: new RegExp("^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=)?$"),
+  isBase64Value(object) {
+    if (typeof object !== 'string') {
+      return false;
+    }
+    return this.base64Pattern.test(object);
+  },
+
   databaseToJSON(object) {
+    let value;
+    if (this.isBase64Value(object)) {
+      value = object;
+    } else {
+      value = object.buffer.toString('base64');
+    }
     return {
       __type: 'Bytes',
-      base64: object.buffer.toString('base64')
+      base64: value
     };
   },
 
   isValidDatabaseObject(object) {    
-    return (object instanceof mongodb.Binary);
+    return (object instanceof mongodb.Binary) || this.isBase64Value(object);
   },
 
   JSONToDatabase(json) {