Refactor to use Enumerable#{count,select,find}.
Add some specs for PDF::Reader::RegisterReceiver.
Add missing encoding for new specs.
Merge pull request #89 from splattael/use-enum

Refactor to use Enumerable#{count,select,find}.
avoid unnecessary object allocations
update stale comment

* it's suggested usage was the pdf-reader < 1 API
fix a failing spec for RegisterReceiver#series

* thanks to Peter Suschlik for reporting