(re)Fix Field definition for field not found if field name starts wit…

…h upper case

Resolves #369

Directly assign the key in the case where the "Direct" target strategy is used alongside with "SYSTEM Key".

Currently, an error is thrown because `key` is not in `$this->fieldName` since it is a system field

src/Mapping/DataTarget/Direct.php

@@ -74,7 +74,9 @@ public function assignData(ElementInterface $element, $data): void
     {
         $setterParts = explode('.', $this->fieldName);
 
-        if (count($setterParts) === 1) {
+        if ($this->fieldName === 'key') {
+            $this->doAssignData($element, $this->fieldName, $data);
+        } elseif (count($setterParts) === 1) {
             //direct class attribute
             $getter = 'get' . ucfirst($this->fieldName);
             if (!$this->checkAssignData($data, $element, $getter)) {