Specify encoding when reading XML references

[ehenneken]

When reading references from the file containing the raw XML references, no encoding is specified, while this is explicitly done for the other types of references, like e.g. TXT references.

TXT references:

pairs = self.get_references(filename=filename, encoding=encoding)

XML references:

pairs = self.get_references(filename=filename)

In practice this is not a real problem, because the default encoding for the get_references function for XML references is UTF-8, while for e.g. TXT references this function has ISO-8859-1 as default encoding. Since our general encoding of reference files is UTF-8, only in non-XML cases the default encoding of get_references needs to be changed.

However, for consistency, the call pairs = self.get_references(filename=filename) for XML references in principle needs to be able to overrride the default encoding.