viewSource gets correct source for replies to PMs

modules['commentPreview'].viewSource, when attempting to view source for /message/messages/, was grabbing the source for the first comment in the thread.  Instead, it should follow the same process as modmail.

http://www.reddit.com/r/RESissues/comments/1949rr/bug_found_an_obscure_and_weird_bug_regarding_the/