Updates reset_counters to allow counter name in params

This updates the CounterCache::ClassMethods#reset_counters method to accept counter names in the counters param, in addition to what it already allows. Right now it only accepts the has_many association name, which is inconsistent with the other methods in this module (all accept counter name) and the docs for this param (they are a little vague, "One or more association counters to reset").

thanks!

/cc @arthurnn