Issue title: how to make two columns as default sorting columns in primeng datatable
 Issue body: how to make two columns as default sorting columns in primeng datatable
<p-dataTable [value]="itemList" [emptyMessage]="itemErrorMessage" scrollable="true"	
		[rows]="50" [rowsPerPageOptions]="[50, 75, 100]" [pageLinks]="3" [multiSortMeta]="[{field: 'createDate', order: -1}, {field: 'ID', order: -1}]"
		[paginator]="itemList.length > 50" [responsive]="true" [scrollHeight]="scrollableValue" sortMode="multiple" (onSort)="changeSort($event)">
Anyone help me!!!!!!!!!
 Comments: 
 Comment 0: Please use forum for support requests, you need to use multiSortMeta, documentation has information already.
{'source': 'GithubIssues', 'repo_name_with_owner': 'primefaces/primeng', 'issue_id': 275374945, 'repo_stars': 8342.0}