Add warning if cookie size exceeds 4000 bytes

[Resonance1584]

Add warning if cookie size exceeds 4000 bytes

[dougwilson]

If the limit is determined by the user-agent, how do we know if we exceeded the limit?

[dougwilson]

We also cannot just console.warn in this code. For example, what if people want to disable that warning, or send that warning to a logging infrastructure? If there is a standard warning framework we can use? Or perhaps if we just use the debug module would that be good enough?

[Resonance1584]

Perhaps just a note in the readme is fine? I assume this is a rare bug - it should really be the responsibility of the user-agent to warn.

[dougwilson]

Gotcha. In order to add a note in the readme, the note needs to be very specific, like exactly what lengths for exactly which browsers? Can you put together a PR?