Fix client.rooms to set room's server_url correctly

If try to fetch room's history through fetched rooms object like

    client.rooms.find {|room| room.name =~ /Hipchat/ }.history

fails with

    URI::InvalidURIError: the scheme http does not accept registry part: :80 (or bad hostname?)

This is because `HipChat::Room.base_uri` is not set at `HipChat::Room#initialize`.

This commit fix above issue.