[4.0] Cassiopeia module header class (#31771)

* [4.0] Cassiopeia module header class

Fixes a silly little error to make the module header class work as intended.

### To test
Create a module and publish it in the sidebar-right position
In the module options give it a header class eg broken

### before
`<h3 class="brokencard-header broken "`

### After
`<hr class="card-header broken"`

* tidy

* tidy

Author: brianteeman

templates/cassiopeia/html/layouts/chromes/card.php

@@ -33,7 +33,7 @@
 	$headerAttribs['id']             = 'mod-' . $module->id;
 
 	if ($headerClass !== 'card-title') :
-		$headerAttribs['class'] .= 'card-header ' . $headerClass;
+		$headerAttribs['class'] = 'card-header ' . $headerClass;
 	endif;
 else:
 	$moduleAttribs['aria-label'] = $module->title;