src/irclib.py: set caps in Irc.__init__

This allows other plugins to request capabilities in a straightforward
way.
Fix test.
src/irclib.py: use a set instead of a list for Irc.caps
Merge pull request #809 from nyuszika7h/irc-caps

src/irclib.py: set caps in Irc.__init__
Fix previous commit.