don't add trailing = to ISUPPORT tokens when value is empty string

[jesopo]

fixes #1069

[slingamn]

Thanks!

So, first I want to confirm with Dan that we want to make this change. But if we're doing it, it would be cool if we could clean up the whole use of *string here, and make Tokens a map[string]string.

[slingamn]

Delightful, I'll merge this next week pending Dan's approval.

[DanielOaks]

Coolcool, yeah they're semantically the same so should be fine. Cleaner so let's do it~