XML writer / parser does not implement attribute normalization, it is not conformant

[bubnikv]

boost::property_tree is able to read the XML attribute values it writes, however it seems to not implement XML attribute normalization:
https://www.w3.org/TR/REC-xml/#AVNormalize

Namely, if a XML attribute value contains newlines or tabs, these newlines or tabs are replaced by libexpat by spaces after parsing and before passing to an application due to the mandatory normalization as shown by the following table from the XML norm:

Please note that an attribute type is considered CDATA by default.

And by the way, none of the XML parsers / writers that I checked (boost serialization, tinyxml, lib3mf) seem to implement normalization.

This issue is similar to 3MFConsortium/lib3mf#288