Fix post -u bug that uses the incorrect summary.

Fix post -u bug that uses the incorrect summary.

If a draft exists for the review request, post -u should use the draft's
summary and description instead of the review request's summary and description
to detect and prompt for the review request to update. This patch fixes the
issue where post -u would always use the review request’s summary and
description even if a draft exists.

Testing done:
Tested with published and draft review requests.
Unit tests passed.

Reviewed at https://reviews.reviewboard.org/r/5041/