QDomDocument::toString() asserts when called on document which specifies an unknown encoding

QDomDocument::toString() asserts when called on a document which specifies an unknown encoding.

The following example reproduces the assertion:

QString xml("<?xml version='1.0' encoding='unknown-encoding'?>"
"<foo>"
" <bar>How will this sentence be handled?</bar>"
"</foo>");

QDomDocument d;
d.setContent(xml);

QString string = d.toString(); // causes assertion

Autotest added: 3e23d922c6547f7295c425b28a9c9fc0791dbfa5