QNetworkRequest: make the default ctor implicit

As in QVarLengthArray (c34242c679aaea6ee1badf6c1e5f274f925f5f50), this will
probably bite someone someday, so fix it before there's a bug report.

Use ctor delegation to keep code size increase small.

There's also the benefit that default-constructing a QNetworkRequest now
no longer creates an expensive QUrl object just to destroy it unused again.

Add an auto-test.

Change-Id: I5ceb5402ca3946048d695244d1b1c36564a1e80a
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/network/access/qnetworkrequest.cpp

@@ -467,6 +467,17 @@ public:
     QString peerVerifyName;
 };
 
+/*!
+    Constructs a QNetworkRequest object with no URL to be requested.
+    Use setUrl() to set one.
+
+    \sa url(), setUrl()
+*/
+QNetworkRequest::QNetworkRequest()
+    : d(new QNetworkRequestPrivate)
+{
+}
+
 /*!
     Constructs a QNetworkRequest object with \a url as the URL to be
     requested.
@@ -474,7 +485,7 @@ public:
     \sa url(), setUrl()
 */
 QNetworkRequest::QNetworkRequest(const QUrl &url)
-    : d(new QNetworkRequestPrivate)
+    : QNetworkRequest()
 {
     d->url = url;
 }

src/network/access/qnetworkrequest.h

@@ -128,7 +128,8 @@ public:
     };
 
 
-    explicit QNetworkRequest(const QUrl &url = QUrl());
+    QNetworkRequest();
+    explicit QNetworkRequest(const QUrl &url);
     QNetworkRequest(const QNetworkRequest &other);
     ~QNetworkRequest();
     QNetworkRequest &operator=(QNetworkRequest &&other) noexcept { swap(other); return *this; }

tests/auto/network/access/qnetworkrequest/tst_qnetworkrequest.cpp

@@ -41,6 +41,7 @@ class tst_QNetworkRequest: public QObject
 private slots:
     void ctor_data();
     void ctor();
+    void implicitDefaultCtor();
     void setUrl_data();
     void setUrl();
     void setRawHeader_data();
@@ -78,6 +79,12 @@ void tst_QNetworkRequest::ctor()
     }
 }
 
+void tst_QNetworkRequest::implicitDefaultCtor()
+{
+    QNetworkRequest r = {};
+    Q_UNUSED(r);
+}
+
 void tst_QNetworkRequest::setUrl_data()
 {
     ctor_data();