Fixed parsing of XML by adding application/xml content type.

Using $response->getHeader('content-type') instead of $responseBody->getContentType() as the later does not report the content type correctly.

E.g. the following request did not work before this change:

https://jenkins.opengeosys.org/job/OGS-6/job/ufz/job/master/api/xml?pretty=true&tree=builds[displayName,url,keepLog]&exclude=/workflowJob/build[keepLog="false"]