Document and further test that QJsonObject::keys() is sorted.

Change-Id: I6b145c1240cce85ad3fea6fb90ddbed629487f83
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

src/corelib/json/qjsonobject.cpp

@@ -223,6 +223,8 @@ QVariantMap QJsonObject::toVariantMap() const
 
 /*!
     Returns a list of all keys in this object.
+
+    The list is sorted lexographically.
  */
 QStringList QJsonObject::keys() const
 {

tests/auto/corelib/json/tst_qtjson.cpp

@@ -1051,6 +1051,8 @@ void tst_QtJson::keySorting()
     QCOMPARE(it.key(), QLatin1String("A"));
     ++it;
     QCOMPARE(it.key(), QLatin1String("B"));
+
+    QCOMPARE(o.keys(), QStringList() << QLatin1String("A") << QLatin1String("B"));
 }
 
 void tst_QtJson::undefinedValues()