If the window.gon variable is already defined, use it instead of overwriting it.

Use case: I'm using a custom namespace `window.TOUT`.  But I may or may not already have `window.TOUT` initialized already.  This pull request gives the same default value, but doesn't overwrite it if it's already set.

If you think this feature is useful, then there are two questions:
1) Is this feature always present or is it configurable?
2) If it's configurable, is it on or off by default.

I can see a case where this change *could* be a breaking change, but that would be a fairly poor implementation where they set a global variable use it, then allow it to be reset by gon and then use that.

I can really go either way on questions 1 or 2, so I thought I'd just ship the easiest option first, get your thoughts on it, and then improve based on your feedback.

Thanks,
Tom