Refs #482: fixes issues in the module utils

- Type of window is "window", not "domwindow" in Gecko
- The name of an exception for a syntax error is "SyntaxError"  in gecko
- fixes utils.mergeObjects: it seems that the object constructor is undefined
  because of a bug in the security layer of sandboxes or a bug in slimerjs?
  Didn't really found the origin of this issue.