change method drop to dropColumn

When using drop() it should drop the whole table, you only want to drop a column

Author: rvzug

src/LaravelArchivableServiceProvider.php

@@ -29,7 +29,7 @@ protected function configureMacros()
         });
 
         Blueprint::macro('dropArchivedAt', function ($column = 'archived_at') {
-            return $this->drop($column);
+            return $this->dropColumn($column);
         });
     }