We would need to escape the attributes, but there's no builtin method
that we can call in libxml2 to do so in a way consistent with the
attribute escape rules and expat.
In fact, expat just repeats the input, while we reconstruct it.
To fix the issue, and fix consistency with expat, we repeat the input as
well. This works by seeking to the start and end of the tag and passing
it to the default handler. This is fine for the parser because the
parser used in ext/xml is always in non-progressive mode, so we have
access to the entire input buffer.
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