Fix issue #7350: if w or h is -1, set width or height to 'auto'

When w or h is -1, then instead of not changing width or height, it
should be set to its preferred size.

src/Wt/WPaintedWidget.C

@@ -354,8 +354,12 @@ DomElement *WPaintedWidget::createDomElement(WApplication *app)
 			"""var u = $(self).find('canvas, img');"
 			"""if (w >= 0) "
 			""  "u.width(w);"
+                        """else "
+                        ""  "u.width('auto');"
 			"""if (h >= 0) "
 			""  "u.height(h);"
+                        """else "
+                        ""  "u.height('auto');"
 			"}");
   }