Initial sync creates _id index even if collection does not have one

var name = 'initial_sync_index_creation'; var replSet = new ReplSetTest({ name: name, nodes: 1, });   replSet.startSet(); replSet.initiate(); var primary = replSet.getPrimary();   primary.getDB('test').runCommand({create:'foo', autoIndexId:false}) primary.getDB('test').foo.insert({a:1}) assert.eq(primary.getDB('test').foo.getIndexes().length, 0);   // Add a secondary node but make it hang before copying databases. var secondary = replSet.add(); secondary.setSlaveOk(); replSet.reInitiate(); replSet.awaitSecondaryNodes(200 * 1000);   assert.eq(primary.getDB('test').foo.getIndexes().length, 0); assert.eq(primary.getDB('test').foo.count(), 1); assert.eq(secondary.getDB('test').foo.count(), 1); assert.eq(secondary.getDB('test').foo.getIndexes().length, 0);