ObjectifiedElement.__setitem__ doesn't honor empty namespace

[KT]

Assignment to a non-namespaced element inside a namespaced element doesn't seem to work:

XML:

    <aaa:Parent xmlns:aaa="urn:aaa">
     <Child>foo</Child>
    </aaa:Parent>

Code:

    root['{}Child'] = 'bar'

Expected result:

    <aaa:Parent xmlns:aaa="urn:aaa">
      <Child>bar</Child>
    </aaa:Parent>

Actual result:

    <aaa:Parent xmlns:aaa="urn:aaa">
      <Child>foo</Child>
      <aaa:Child>bar</aaa:Child>
    </aaa:Parent>

Python : (2, 7, 6, 0)
lxml.etree : (3, 4, 1, 0)
libxml used : (2, 9, 0)
libxml compiled : (2, 9, 0)
libxslt used : (1, 1, 28)
libxslt compiled : (1, 1, 28)