Report redis plugin as "redis"

Hey,

Have been playing around with the hiredis based plugin from the PR you put in to the main collectd repo and noticed that it reports metrics using the "type" twice, e.g:

`hostname.rdb_changes_since_last_save.all.rdb_changes_since_last_save 2.000000 1377782863`

For consistency with the way the other plugins submit data (and with the old credis based plugin), I think the type should be set to "redis" rather than `type`, do you agree (or am I wrong)? PR fixes this.