Fixing issue #18 - Last chunk not processed when the last line has no non-empty values

## Fixing issue #18

- Calling `next if hash.empty? if options[:remove_empty_hashes]` in
  the `while ! f.eof?` loop caused the subsequent lines, including
  `yield chunk` to be skipped in the event `f.eof?`, `hash.empty?` and
  `options[:remove_empty_hashes]` were all true, causing the final
  chunk to be discarded.

- Added very small MiniTest spec suite.