ENHANCEMENT: SSF-124 make GridFieldSortableHeader to show the extra classes for each of the header field.

forms/gridfield/GridFieldSortableHeader.php

@@ -85,6 +85,7 @@ class GridFieldSortableHeader implements GridField_HTMLProvider, GridField_DataM
 				}
 			} else {
 				$field = new LiteralField($columnField, '<span class="non-sortable">' . $title . '</span>');
+				$field->addExtraClass("col-".preg_replace('/[^\w]/', '-', $columnField));
 			}
 			$forTemplate->Fields->push($field);
 		}

templates/Includes/GridFieldSortableHeader_Row.ss

@@ -1,5 +1,5 @@
 <tr class="sortable-header">
 	<% control Fields %>
-		<th class="main"><span>$Field</span></th>
+		<th class="main $extraClass"><span>$Field</span></th>
 	<% end_control %>
 </tr>