#940 Bibtexml export produces invalid document

[Luiz Camara]

If the bibtexkey contains characters that must be encoded as xml entities like & it produces invalid documents in the bibtexkey field and in the nstandard (file) field.

The attached patch fix the problem in the bibtexkey field.

Unfortunately i could not find a way to fix the nstandard field.
If i put XMLChars before WrapFileLinks it will output two tags with mangled output
If i put XMLChars after WrapFileLinks it will output the open and close tag encoded as entities thus not being recognizing as a node when parsed

Attached is a
An bibtex file with example

[Luiz Camara]

Partial patch (fixes bibtextkey)

[Luiz Camara]

Bibtex example with bibtextkey with xml markup