Driver#html returns raw data for non-HTML content

The behavior of QWebFrame::toPlainText is undefined if the content
contains non-Latin1 characters. In Qt 5, additional MIME types are
considered supported, so there's no way to get at the raw data when
unsupported content has been loaded. Instead, proxy network replies
through NetworkReplyProxy which provides repetitive reads.
JavaScript line numbers off by one in Qt 5