Fix phpstan errors

Author: Art4

tests/Functional/ParsingTest.php

@@ -656,7 +656,9 @@ public function testParseNormativeStatementsForVersion10Correctly(): void
         $this->assertInstanceOf('Art4\JsonApiClient\V1\Document', $document);
         $this->assertSame(['data', 'included', 'jsonapi'], $document->getKeys());
         $this->assertSame('1.0', $document->get('jsonapi.version'));
+        $this->assertInstanceOf(Accessable::class, $document->get('data'));
         $this->assertCount(6, $document->get('data')->getKeys());
+        $this->assertInstanceOf(Accessable::class, $document->get('included'));
         $this->assertCount(184, $document->get('included')->getKeys());
     }
 
@@ -668,7 +670,9 @@ public function testParseNormativeStatementsForVersion11Correctly(): void
         $this->assertInstanceOf('Art4\JsonApiClient\V1\Document', $document);
         $this->assertSame(['data', 'included', 'jsonapi'], $document->getKeys());
         $this->assertSame('1.1', $document->get('jsonapi.version'));
+        $this->assertInstanceOf(Accessable::class, $document->get('data'));
         $this->assertCount(6, $document->get('data')->getKeys());
+        $this->assertInstanceOf(Accessable::class, $document->get('included'));
         $this->assertCount(188, $document->get('included')->getKeys());
     }
 
@@ -679,8 +683,9 @@ public function testParseJsonApiObjectWithVersion11Correctly(): void
 
         $this->assertInstanceOf('Art4\JsonApiClient\V1\Document', $document);
         $this->assertSame(['meta', 'jsonapi'], $document->getKeys());
+        $this->assertInstanceOf(Accessable::class, $document->get('jsonapi'));
         $this->assertSame(['version'], $document->get('jsonapi')->getKeys());
-        // TODO: Add support for unknown properties
+        // TODO #90: Add support for unknown properties
         // $this->assertSame(['version', 'ext', 'profile'], $document->get('jsonapi')->getKeys());
         $this->assertSame('1.1', $document->get('jsonapi.version'));
     }