dashdemux: copy ContentProtection element including xml namespaces
Commit bc09d8cc changed gstmpdparser to put the entire 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.