Fix the issue that attachment can not be saved when using firefox nightly

Firefox introduces a new parameter on nsIWebBrowserPersist.saveURI function.
The attach function is broken due to that change.
1) This can go in Zotero.Utilities.Internal, since it won't be used outside of Firefox/Standalone.

2) The Fx <18 code can be removed, so this should only need one ```try```/```catch```.

3) Code style needs adjusting.

I think this should work:

```javascript
saveURI: function (wbp, source, target) {
	// Firefox 35 and below
	try	{
		wbp.saveURI(source, null, null, null, null, target, null);
	}
	// Firefox 36+ needs one more parameter
	catch (e if e.name === "NS_ERROR_XPC_NOT_ENOUGH_ARGS") {
		wbp.saveURI(source, null, null, null, null, null, target, null);
	}
}
```