feat(module:datepicker): support string and number type as value (#593)

change NzDatePipe to support display string type in valid ISO 8601 
format (eg: '2013-02-08 09:30:26.123') and number type (eg: 
timestamp-'123456789') .

修改了NzDatePipe从而支持显示输入的ISO 8601时间格式的字符串类型（如： “2013-02-08 09:30:26.123”）和数字类型（如：时间戳－“123456789”）。

src/components/util/nz-date.pipe.ts

@@ -3,11 +3,11 @@ import * as moment from 'moment';
 
 @Pipe({ name: 'nzDate' })
 export class NzDatePipe implements PipeTransform {
-  transform(value: Date, formatString: string): string {
-    if (value) {
-      return moment(+value).format(formatString);
+  transform(value: Date | number | string, formatString: string): string {
+    if (moment(value).isValid()) {
+      return moment(value).format(formatString);
     } else {
-      return '';
+      return 'Invalid Date';
     }
   }
 }