Ugly procedure to remove XML processing instructions (PI) from metadata.
This method is designed to handle the most likely scenario: one supplies a complete XML document as metadata, and
the document starts with a PI:
<?xml version="1.0" encoding="UTF-8"?>
<McMurdoDryValleys>
<FieldPhotos>
<FieldPhoto>
<!-- and so on -->
</FieldPhoto>
</FieldPhotos>
</McMurdoDryValleys>
It is helpful to remember that XStream is processing a stream of XML, and when this MetadataConverter is invoked,
XStream is somewhere in the middle of the stream. If the underlying XML parser encounters a PI from inline
metadata, things hit the hizzy. So PIs need to be removed from inline XML metadata, either prior to parsing,
or during parsing.