Remove `?subject=unsubscribe` from mailto addresses

I found an email in my inbox which contained this as one of the entries in its `List-Unsubscribe` header:

```text
unsubscribe-blahblahblah@mailin1.us2.mcsv.net?subject=unsubscribe
```

Unless the mail gem strips these query strings from the mailto links, we'll be screwed. This PR strips any query strings from the end of these links.