Remove trainling dot from download attibution text to not break URLs

When formatting the attribution as plain text, make sure that the
source parameter (which typically ends with an URL) is not followed
by a '.', which could break autolinking when e.g. the text is
copied into a mail.

Change-Id: I5406fe3a55bc4ef6802b32d5a3d533ce9f49b361
Merge "Remove trainling dot from download attibution text to not break URLs"