Allow numeric headers in Sidebar

A numeric string set as key of an array gets casted to an integer. Since
I458c0af3114754ddf3d721f6c374e249f482e4cf wfMessage does not except
integer as message key, therefore cast the heading name back to string.

Bug: T73639
Change-Id: I376fb49d03251a71372738c531a09f014540b958

VectorTemplate.php

@@ -282,6 +282,9 @@ class VectorTemplate extends BaseTemplate {
 				continue;
 			}
 
+			// Numeric strings gets an integer when set as key, cast back - T73639
+			$name = (string)$name;
+
 			switch ( $name ) {
 				case 'SEARCH':
 					break;