Replace the whole subject of the Invitation email if the user types in a new one

Currently, if the "LET USERS PERSONALIZE MESSAGES" option is enabled by an Admin from http://localhost:8080/xwiki/bin/admin/XWiki/XWikiPreferences?admin=globaladmin&section=Invitation, users can personalize both the Subject and the Message of the email.

If an user types in something in the Subject field, the expected behaviour is that what was typed in is going to be the new email subject, but that's not the case. The written text is simply appended to the already existing "Admin has invited you to join localhost".