Dutch end of email is not detected correctly

This part of the gem: `if text =~ /^(?!On.*On\s.+?wrote:)(On\s(.+?)wrote:)$/nm` works correctly for non-Dutch emails, but this is not correctly parsed as the end of the email:

`Op zondag 31 mei 2015 heeft Name het volgende geschreven:`

It would be awesome if this could be added!

Search the gem for the shown `On ... wrote:` regular expression and inspect the email parsing path around it. Add coverage for the Dutch `Op zondag ... heeft ... het volgende geschreven:` example, then verify that it is detected as the end of the email without breaking the existing non-Dutch behavior.

Vom Indexierungsmodell aus dem Issue-Text verfasst.