OAI plugin: Fix encoding in XML declaration

According to the OAI PMH protocol version 2.0, chapter 3.2
(XML Response Format), the "first tag output is an XML declaration
where the version is always 1.0 and the encoding is always UTF-8".

Goobi used utf-8 instead of UTF-8 and at least one validator complains
because of that difference.

Signed-off-by: Stefan Weil <sw@weilnetz.de>

dlf/plugins/oai/class.tx_dlf_oai.php

@@ -415,7 +415,7 @@ class tx_dlf_oai extends tx_dlf_plugin {
 		$this->deleteExpiredTokens();
 
 		// Create XML document.
-		$this->oai = new DOMDocument('1.0', 'utf-8');
+		$this->oai = new DOMDocument('1.0', 'UTF-8');
 
 		// Add processing instruction (aka XSL stylesheet).
 		if (!empty($this->conf['stylesheet'])) {