This will allow the Metadata to be successfully extracted from the PDF file in issue 10395.
Furthermore, this patch also fixes a bug in `Metadata.get` which causes the method to return `null` rather than an empty string or zero (since either ought to be allowed).
The error was triggered for a particular set of metadata, where an end tag was encountered without the corresponding begin tag being present in the data.
(The patch also fixes a minor oversight, from a recent PR, in the `SimpleDOMNode.nextSibling` method.)
With the new XML parser, see PR 9573, the referenced PDF file now causes `getMetadata` to fail when incomplete XML tags are encountered. This provides a simple, and hopefully generally useful, work-around that may also help prevent future bugs.
(Without being able to reproduce nor even understand the other (non XML) errors mentioned in issue 8884, I'd say that this patch is enough to close that one as fixed.)
Please note that while this could be considered a regression in user-facing behaviour, I'm not convinced that it's really a regression as such since prior to PR 8912 the Metadata would fail to parse (with an XML error) and thus be ignored when setting the viewer title.
With the refactored Metadata parsing we're now able to parse this, which uncovered issues with a subset of broken Ghostscript Metadata that uses HTML character names.
Fixes https://bugzilla.mozilla.org/show_bug.cgi?id=1424938
The `DOMParser` is most likely overkill and may be less secure.
Moreover, it is not supported in Node.js environments.
This patch replaces the `DOMParser` with a simple XML parser. This
should be faster and gives us Node.js support for free. The simple XML
parser is a port of the one that existed in the examples folder with a
small regex fix to make the parsing work correctly.
The unit tests are extended for increased test coverage of the metadata
code. The new method `getAll` is provided so the example does not have
to access internal properties of the object anymore.
