[JsonStreamer] Fix encoding iterable lists

Author: mtarld

src/Symfony/Component/JsonStreamer/Tests/JsonStreamWriterTest.php

@@ -105,7 +105,7 @@ public function testWriteCollection()
         );
 
         $this->assertWritten(
-            '{"0":{"id":1,"name":"dummy"},"1":{"id":1,"name":"dummy"}}',
+            '[{"id":1,"name":"dummy"},{"id":1,"name":"dummy"}]',
             new \ArrayObject([new ClassicDummy(), new ClassicDummy()]),
             Type::iterable(Type::object(ClassicDummy::class), Type::int()),
         );

src/Symfony/Component/JsonStreamer/Write/PhpAstBuilder.php

@@ -294,7 +294,9 @@ private function buildYieldStatements(DataModelNodeInterface $dataModelNode, arr
         if ($dataModelNode instanceof CollectionNode) {
             ++$context['depth'];
 
-            if ($dataModelNode->getType()->isList()) {
+            $collectionKeyType = $dataModelNode->getType()->getCollectionKeyType();
+
+            if ($collectionKeyType instanceof BuiltinType && TypeIdentifier::INT === $collectionKeyType->getTypeIdentifier()) {
                 return [
                     new Expression(new Yield_($this->builder->val('['))),
                     new Expression(new Assign($this->builder->var('prefix'), $this->builder->val(''))),