entities as attribute values

[scoder]

I'm trying to write an xml file that has entities as values for attributes. If I try to write the entity as text, the & is xml-escaped to &. If I try to call set() with an Entity, I get an error:

    >>> import lxml.etree
    >>> tree= lxml.etree.fromstring ('<a foo="bar"/>')
    >>> tree.set ('foo', '&bar;')
    >>> print lxml.etree.tostring (tree)
    <a foo="&amp;bar;"/>
    >>> tree.set ('foo', lxml.etree.Entity ('bar'))
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "lxml.etree.pyx", line 695, in lxml.etree._Element.set (src/lxml/lxml.etree.c:35527)
      File "apihelpers.pxi", line 563, in lxml.etree._setAttributeValue (src/lxml/lxml.etree.c:15481)
      File "apihelpers.pxi", line 1367, in lxml.etree._utf8 (src/lxml/lxml.etree.c:22126)
    TypeError: Argument must be bytes or unicode, got '_Entity'

Originally reported by Marcos Dione.