support query alias for the mapped attribute

Author: jarektkaczyk

src/Mappable.php

@@ -89,7 +89,7 @@ protected function mappedSelect(Builder $query, ArgumentBag $args)
         $columns = $args->get('columns');
 
         foreach ($columns as $key => $column) {
-            list($column) = $this->extractColumnAlias($column);
+            list($column, $as) = $this->extractColumnAlias($column);
 
             // Each mapped column will be selected appropriately. If it's alias
             // then prefix it with current table and use original field name
@@ -107,6 +107,10 @@ protected function mappedSelect(Builder $query, ArgumentBag $args)
 
                 $columns[$key] = "{$table}.{$mapped}";
 
+                if ($as !== $mapped) {
+                    $columns[$key] .= " as {$as}";
+                }
+
             // For non mapped columns present on this table we will simply
             // add the prefix, in order to avoid any column collisions,
             // that are likely to happen when we are joining tables.