QRestReply: micro-optimize optional<QStringDecoder> handling

Use optional::emplace() instead of assignment from an rvalue.

Saves the move constructor and destructor calls, both of which are not
trivial (due to cleanFn).

Change-Id: Ief77626c77d0c8c2ce17e3b1a21496a7da969761
Reviewed-by: Juha Vuolle <juha.vuolle@qt.io>
(cherry picked from commit 063e31209b1750b6cbd2c3f57c49ce372b13a91a)
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/network/access/qrestreply.cpp

@@ -173,7 +173,7 @@ QString QRestReply::readText()
 
     if (!d->decoder) {
         const QByteArray charset = QRestReplyPrivate::contentCharset(wrapped);
-        d->decoder = QStringDecoder(charset);
+        d->decoder.emplace(charset);
         if (!d->decoder->isValid()) { // the decoder may not support the mimetype's charset
             qCWarning(lcQrest, "readText(): Charset \"%s\" is not supported", charset.constData());
             return result;