HttpTestServer: pass std::function by value and move into place

Virtually all callers of this function (will) pass rvalues, so take
the std::function by value (reaping C++17 guaranteed copy elision) and
std::move() into the member variable ("perfect sink").

Like for many owning types, moves are much cheaper than copies for
std::function, because the external state is merely tranferred between
objects, and not copied.

Amends e560adef213301318dcc13d4db155624846e0420.

Change-Id: I269b54e51ba09ac595ac4e4f255209778819adad
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
(cherry picked from commit a56461883830cecf281b1d6db5e7d6103154d3d9)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

tests/auto/network/access/qrestaccessmanager/httptestserver.cpp

@@ -31,10 +31,6 @@ QUrl HttpTestServer::url()
     return QUrl(u"http://127.0.0.1:%1"_s.arg(serverPort()));
 }
 
-void HttpTestServer::setHandler(const Handler &handler) {
-    m_handler = handler;
-}
-
 void HttpTestServer::handleConnected()
 {
     Q_ASSERT(!m_socket); // No socket must exist previously, this is a single-connection server

tests/auto/network/access/qrestaccessmanager/httptestserver_p.h

@@ -9,6 +9,8 @@
 #include <QtCore/qmap.h>
 #include <QtCore/qurl.h>
 
+#include <functional>
+
 // This struct is used for parsing the incoming network request data into, as well
 // as getting the response data from the testcase
 struct HttpData {
@@ -72,7 +74,7 @@ public:
     // Settable callback for testcase. Gives the received request data, and takes in response data
     using Handler = std::function<void(const HttpData &request, HttpData &response,
                                        ResponseControl &control)>;
-    void setHandler(const Handler &handler);
+    void setHandler(Handler handler) { m_handler = std::move(handler); }
 
 private slots:
     void handleConnected();