[xml] htmlOmittedDefaultValue is not thread-aware

[Stefan Behnel <stefan_ml behnel de>]

Hi,

I noticed that htmlOmittedDefaultValue is not thread aware, so it can't be
used in multi-threaded environments to parse HTML fragments instead of HTML
documents. Is there any other way to parse fragments? Or could this option be
changed to work on a per-thread basis?

Stefan