Plugins: Add a missing space between classes on `<td>` element for custom columns of the Plugins list table.

Props crstauf.
Fixes #37460.

git-svn-id: https://develop.svn.wordpress.org/trunk@38149 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/class-wp-plugins-list-table.php

@@ -816,7 +816,7 @@ class WP_Plugins_List_Table extends WP_List_Table {
 					echo "</div></td>";
 					break;
 				default:
-					$classes = "$column_name column-$column_name$class";
+					$classes = "$column_name column-$column_name $class";
 
 					echo "<td class='$classes{$extra_classes}'>";