Init .disconnectedSince to null (not undefined)

As discussed in issue #77, user.connected() is broken unless we differentiate values of null vs undefined in .disconnectedSince
Null out user.disconnectedSince, don't delete

As discussed in issue #77, differentiating null vs undefined value for .disconnectedSince is important to user.connected() strict equality test
Merge pull request #79 from Markleford/patch-1

Fix strict equality test in user.connected()