input/collectd: Rename @event to @collectd

  @untergeek made a good point on @event being an internal logstash
  variable that could clash.
input/collectd: Various bugfixes

  * It didn't generate an event for the last event in the packet
  * Raise an exception if the authfile is not found
  * Removed some debug statements
  * Correct usage of 'insist' in rspec