Comment 2 for bug 1424232

The bug stems from a non-cleaned-up nsmap-property:

>>> x = etree.fromstring('<div xmlns="foo"/>')
>>> x.tag = 'div'
>>> assert etree.fromstring(etree.tostring(x)).tag == 'div'
Traceback (most recent call last):
  File "<input>", line 1, in <module>
AssertionError

>>> x.nsmap
{None: 'foo'}