issue #2 - use constant whenever possible

Author: mikaelcom

src/Tag/AbstractTagOperationElement.php

@@ -36,7 +36,7 @@ public function getMessage(): ?TagMessage
         $messageName = $this->getAttributeMessage();
 
         if (!empty($messageName)) {
-            $message = $this->getDomDocumentHandler()->getElementByNameAndAttributes('message', [
+            $message = $this->getDomDocumentHandler()->getElementByNameAndAttributes(AbstractDocument::TAG_MESSAGE, [
                 'name' => $messageName,
             ], true);
         }

src/Tag/TagAttributeGroup.php

@@ -4,20 +4,22 @@
 
 namespace WsdlToPhp\WsdlHandler\Tag;
 
+use WsdlToPhp\WsdlHandler\AbstractDocument;
+
 class TagAttributeGroup extends Tag
 {
     public function getReferencingElements(): array
     {
         $elements = [];
-        $attributeGroups = $this->getDomDocumentHandler()->getElementsByNameAndAttributes('attributeGroup', [
+        $attributeGroups = $this->getDomDocumentHandler()->getElementsByNameAndAttributes(AbstractDocument::TAG_ATTRIBUTE_GROUP, [
             'ref' => sprintf('*:%s', $this->getAttributeName()),
         ]);
         /*
          * In case of a referencing element that use this attributeGroup that is not namespaced,
          * use the non namespaced value
          */
         if (empty($attributeGroups)) {
-            $attributeGroups = $this->getDomDocumentHandler()->getElementsByNameAndAttributes('attributeGroup', [
+            $attributeGroups = $this->getDomDocumentHandler()->getElementsByNameAndAttributes(AbstractDocument::TAG_ATTRIBUTE_GROUP, [
                 'ref' => sprintf('*%s', $this->getAttributeName()),
             ]);
         }

src/Tag/TagMessage.php

@@ -4,11 +4,13 @@
 
 namespace WsdlToPhp\WsdlHandler\Tag;
 
+use WsdlToPhp\WsdlHandler\AbstractDocument;
+
 class TagMessage extends Tag
 {
     public function getPart(string $name): ?TagPart
     {
-        return $this->getChildByNameAndAttributes('part', [
+        return $this->getChildByNameAndAttributes(AbstractDocument::TAG_PART, [
             'name' => $name,
         ]);
     }