Add ArgumentError when find receives a nil guid

This would have saved me some debug time, so it's probably similarly useful to an end user.
Refs #10893: Add Helix::Video#download; Specs
Refs #10893: Add Helix::Video#play; Specs
Remove unused csv_text
Fix expectations/mocks for config.build_url inside download/play
Allow Helix::Video#download and #play to accept content_type (for mp3, etc.); Specs
DRY up Helix::Video#download & #play w/#generic_download
Up version from 0.0.2.4.pre -> 0.0.2.5.pre

Helix::Video#download & #play, plus some minor refactorings