[Serializer] add a context key to return csv always as collection

src/Symfony/Component/Serializer/Encoder/CsvEncoder.php

@@ -150,6 +150,10 @@ class CsvEncoder implements EncoderInterface, DecoderInterface
         }
         fclose($handle);
 
+        if ($context['as_collection'] ?? false) {
+            return $result;
+        }
+
         if (empty($result) || isset($result[1])) {
             return $result;
         }

src/Symfony/Component/Serializer/Tests/Encoder/CsvEncoderTest.php

@@ -208,6 +208,22 @@ CSV
         , 'csv'));
     }
 
+    public function testDecodeOnlyOneAsCollection()
+    {
+        $this->encoder = new CsvEncoder(',', '"', '\\', '.');
+
+        $expected = array(
+            array('foo' => 'a'),
+        );
+
+        $this->assertEquals($expected, $this->encoder->decode(<<<'CSV'
+foo
+a
+
+CSV
+            , 'csv', array('as_collection' => true)));
+    }
+
     public function testDecodeToManyRelation()
     {
         $expected = array(