set charset on backend for xml2enc tests

Make backend behavior match testcase assumptions:

    # Backend sends Content-Type: application/xml; charset=utf-8
    ['/doc.xml', "application/xml; charset=utf-8", "fóó\n" ],

    # Backend sends Content-Type: application/foo+xml; charset=utf-8
    ['/doc.fooxml', "application/foo+xml; charset=utf-8", "fóó\n" ],



git-svn-id: https://svn.apache.org/repos/asf/httpd/test/framework/trunk@1916758 13f79535-47bb-0310-9956-ffa450edef68