OAI-PMH harvesting error because of invalid XML output.

[nedbaldessin]

When harvesting the OAI-PMH repository, we get this error after a while :

<?xml version="1.0" encoding="UTF-8"?>
<OAI-PMH xmlns="http://www.openarchives.org/OAI/2.0/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.openarchives.org/OAI/2.0/ http://www.openarchives.org/OAI/2.0/OAI-PMH.xsd">
  <responseDate>2022-09-21T21:32:57Z</responseDate>
  <request>https://collection.mobiliernational.culture.gouv.fr/api/oai-pmh</request>
  <error code="badArgument">DOMDocument::createElementNS(): unterminated entity reference  GILBERT Achille</error>
</OAI-PMH>

To reproduce, use this validator:
https://validator.oaipmh.com/#ListRecords
And put the values in screenshot.

This is probably because the author name is PECQUEREAU Théodore & GILBERT Achille and the & character isn't properly escaped in the XML output.

Properly escaping the output should fix the problem.