Workaround bug in QtXML: Fix xmlToString when QDomElement contains Un…

…icode characters above 0xFFFF

src/xmpp/xmpp-core/xmlprotocol.cpp

@@ -97,6 +97,14 @@ static QString xmlToString(const QDomElement &e, const QString &fakeNS, const QS
 	QString out;
 	{
 		QTextStream ts(&out, QIODevice::WriteOnly);
+		// NOTE: Workaround for bug in QtXML:
+		// Qt by default convert low surrogate to XML notation &#x....; and let high in binary!
+		//
+		// Qt is calling encode function per UTF-16 codepoint, which means that high and low
+		// surrogate pairs are encoded separately. So all encoding except UTF-16 will leads
+		// to damaged Unicode characters above 0xFFFF. Internal QString encoding is UTF-16
+		// so this should be safe as QString still contains valid Unicode characters.
+		ts.setCodec("UTF-16");
 		fake.firstChild().save(ts, 0);
 	}
 	// 'clip' means to remove any unwanted (and unneeded) characters, such as a trailing newline