Invitation app's custom message written on multiple lines isn't properly displayed in the email

Steps to reproduce:

1. Set your SMTP server for the Invitation app: http://localhost:8080/xwiki/bin/admin/XWiki/XWikiPreferences?editor=globaladmin&section=Invitation
2. Go to Invitation.WebHome and type in a message on multiple lines
3. Click Preview -> the custom message appears on a single line
4. Click "Send Mail"

You'll notice in the e-mail that the personalised message isn't written on multiple lines, but separated by a space, just like in Preview mode.