fixed checking for exist property in document parser (#790)

Author: alexander-schranz

Mapping/DocumentParser.php

@@ -439,7 +439,7 @@ private function getDocumentPropertiesReflection(\ReflectionClass $reflectionCla
         $properties = [];
 
         foreach ($reflectionClass->getProperties() as $property) {
-            if (!in_array($property->getName(), $properties)) {
+            if (!array_key_exists($property->getName(), $properties)) {
                 $properties[$property->getName()] = $property;
             }
         }