repair non-namespace-aware attribute mis-interpretation in ElementTreeContentHandler

this pull request allows the usage of non-NS aware SAX parsers to work properly with ElementTreeContentHandler, regarding attributes.   This refers to https://bugs.launchpad.net/lxml/+bug/1136509.

Summary test:

    document = """<?xml version="1.0" encoding="utf-8"?>
    <SomeDocument>
        <Data FooBar="123">
        </Data>
    </SomeDocument>
    """

    from lxml import sax
    from xml.sax import parse
    from StringIO import StringIO

    parse(StringIO(document), sax.ElementTreeContentHandler())

Above, the non-namespace aware parser generated by xml.sax.parse() calls the startElement() method, which ElementTreeContentHandler refers to startElementNS().  The attribute "FooBar" however must be expanded to the form `(None, "FooBar"): "123"` in order to be interpreted correctly by startElementNS().