dashdemux: copy ContentProtection element including xml namespaces

dashdemux: copy ContentProtection element including xml namespaces

Commit bc09d8cc changed gstmpdparser to put the entire
<ContentProtection> element in the "value" field, so that DRMs
other than PlayReady could make use of the data inside this
element.

However, the data in the "value" field does not include any
XML namespace declarations that are used within the element. This
causes problems for a namespace aware XML parser that wants to
make use of this data.

This commit modifies the way the XML is converted to a string
so that XML namespaces are preserved in the output.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2487>