Fixed crash when saving two new maps using the same file name

For new maps, the "reader format" was not being set, which caused
problems when these maps were reloaded, either manually or as a result
of the file changing on disk (which can happen externally or because of
creating another new map and saving it with the same file name).

Closes #1734

src/tiled/documentmanager.cpp

@@ -496,6 +496,7 @@ bool DocumentManager::saveDocumentAs(Document *document)
 
         MapFormat *format = helper.formatByNameFilter(selectedFilter);
         mapDocument->setWriterFormat(format);
+        mapDocument->setReaderFormat(format);
 
     } else if (auto tilesetDocument = qobject_cast<TilesetDocument*>(document)) {
         if (selectedFilter.isEmpty())