RSS Feed: use bbpress functions for post and reply authors

[yoavf]

Currently, the RSS feed output from bbp_display_replies_feed_rss2 uses the_author() to output the display name for the topic and reply authors.

This prevents any filters added to bbpress core functions from being applied to the display names.
We should use bbp_author_display_name() had bbp_reply_author_display_name() instead.