Cast response body to string in binary mode.

QVariantHelper expects body to be the same type as before conversion.
If target type doesn't match the source type, QVariantHelper will return 0.
We can't convert in this way: QString > QVariant -> QByteArray
Instead we must use the following way: QString > QVariant -> QString

Fixes: ariya#13026

src/webserver.cpp

@@ -409,7 +409,7 @@ void WebServerResponse::write(const QVariant& body)
     if (m_encoding.isEmpty()) {
         data = body.toString().toUtf8();
     } else if (m_encoding.toLower() == "binary") {
-        data = body.toByteArray();
+        data = body.toString().toLatin1();
     } else {
         Encoding encoding;
         encoding.setEncoding(m_encoding);