Incorrect URL detection (final slash)

Expected behaviour

URLs automatically converted to links can include a final slash part of the URL.

Actual behaviour

If the URL contains a final slash, the link generated ignores it, breaking some links:

Content-Type: text/html;
 charset=utf-8
Content-Transfer-Encoding: 8bit

<a href="http://google.com">http://google.com</a>/<br>

Steps to reproduce

1. Select HTML format in the settings
2. Send a new mail with the following content: http://google.com

Environment

K-9 Mail version: 5.008

Android version: 6.0

Account type (IMAP, POP3, WebDAV/Exchange): IMAP

[Valodim]

There is already a promising PR for this, see #2392

[a3y3]

I’m new to Open Source, and I’d like to work on this issue!