add test and remove string convert

Author: fuyuki0511

src/Query/Builder.php

@@ -854,7 +854,7 @@ public function pluck($column, $key = null)
         // Convert ObjectID's to strings
         if (((string) $key) === '_id') {
             $results = $results->map(function ($item) {
-                $item->_id = (string) $item->id;
+                $item->_id = $item->id;
 
                 return $item;
             });

tests/Query/BuilderTest.php

@@ -1573,4 +1573,28 @@ private static function getBuilder(): Builder
 
         return new Builder($connection, null, $processor);
     }
+
+    public function testPluckObjectId()
+    {
+        DB::table('users')->insert([
+            ['_id' => $id = new \MongoDB\BSON\ObjectId(), 'name' => 'Jane Doe', 'age' => 20],
+        ]);
+
+        $names = DB::table('users')->pluck('name', '_id')->toArray();
+        $this->assertCount(1, $names);
+        $this->assertArrayHasKey((string) $id, $names);
+        $this->assertEquals([(string) $id => 'Jane Doe'], $names);
+    }
+
+    public function testPluckObjectIdWithIdAlias()
+    {
+        DB::table('users')->insert([
+            ['_id' => $id = new \MongoDB\BSON\ObjectId(), 'name' => 'Jane Doe', 'age' => 20],
+        ]);
+
+        $names = DB::table('users')->pluck('name', 'id')->toArray();
+        $this->assertCount(1, $names);
+        $this->assertArrayHasKey((string) $id, $names);
+        $this->assertEquals([(string) $id => 'Jane Doe'], $names);
+    }    
 }