fix XmlItemExporter in Python 2.7.4 and 2.7.5

Scrapy tests started to fail in Python 2.7.4 and 2.7.5: https://gist.github.com/alexcepoi/6d51b507d9bee4a62d9e

This happened because of a breaking change in 2.7.4. It was [fixed](http://hg.python.org/cpython/rev/a32a3b79f5e8/) in Python hg repository (and will be likely available at some time as 2.7.6), but at the meantime XmlItemExporter is broken.