Fix calling toGMTString on lastModified

[mabrikan]

This fixes #276.

Function checkNotifications is always passed a string value for lastModified (unless, localStore is empty, in which case a Date object is created), so calling toGMTString() resulted in an error.

This change does the following:

1. Fix the bug by removing unneeded toGMTString() call
2. make sure lastModified is always a string, even when localStore is empty
3. Use toUTCString() as it is more consistent with how JS interprets time.

[notlmn]

Thank you 🙌

[mabrikan]

You're most welcome.