Fix invalid PSSH field

[kotori2]

PSSH is supposed to be a sub-tag of ContentProtection, not an attribute. Same thing for the PSSH class.
Example: https://cdn.bitmovin.com/content/assets/art-of-motion_drm/mpds/11331.mpd

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<MPD id="505916a3-fc9b-4088-be98-d90796861a3a" profiles="urn:mpeg:dash:profile:isoff-main:2011" type="static" availabilityStartTime="2015-09-17T09:32:38.000Z" publishTime="2015-09-17T09:32:46.000Z" mediaPresentationDuration="P0Y0M0DT0H3M30.000S" minBufferTime="P0Y0M0DT0H0M1.000S" bitmovin:version="1.6.0" xmlns:ns2="http://www.w3.org/1999/xlink" xmlns="urn:mpeg:dash:schema:mpd:2011" xmlns:bitmovin="http://www.bitmovin.net/mpd/2015">
    <Period>
        <AdaptationSet mimeType="video/mp4" codecs="avc1.42c00d">
            <SegmentTemplate media="../video/$RepresentationID$/cenc_dash/segment_$Number$.m4s" initialization="../video/$RepresentationID$/cenc_dash/init.mp4" duration="4000" startNumber="0" timescale="1000"/>
            <Representation id="180_250000" bandwidth="250000" width="320" height="180" frameRate="25">
                <ContentProtection schemeIdUri="urn:mpeg:dash:mp4protection:2011" value="cenc" cenc:default_KID="eb676abb-cb34-5e96-bbcf-616630f1a3da" xmlns:cenc="urn:mpeg:cenc:2013"/>
                <ContentProtection schemeIdUri="urn:uuid:edef8ba9-79d6-4ace-a3c8-27dcd51d21ed">
                    <cenc:pssh xmlns:cenc="urn:mpeg:cenc:2013">AAAAW3Bzc2gAAAAA7e+LqXnWSs6jyCfc1R0h7QAAADsIARIQ62dqu8s0Xpa7z2FmMPGj2hoNd2lkZXZpbmVfdGVzdCIQZmtqM2xqYVNkZmFsa3IzaioCSEQyAA==</cenc:pssh>
                </ContentProtection>
            </Representation>
...

I also fixed the invalid namespace of cenc:default_KID. I'm sure the ns2 namespace is also not working but I don't know the exact right namespace for that.