check edge cases for ignoring prolines

What happens is a peptides prolines only?
Or shorter than drop_first?
Trailing prolines are up to drop_first?

Start by locating the proline-filtering entry point and checking how drop_first is applied. Reproduce the three cases described: peptides containing only prolines, peptides shorter than drop_first, and trailing prolines relative to drop_first. Done means the behavior is verified and covered for these edge cases.

索引モデルが issue の本文から書いたものです。