cubicweb-blog #2157366 Use publish date in RSS feed [validation pending]

Often, we work on a blogentry before publishing it, sometimes it can be days between the creation of an element and it's publishing. Thus, the RSS item date should use the published date instead of the creation date.

done in1.9.3
load left0.000
closed by#5ad30b94a083 [entities] BlogEntry.dc_date() should return publication date when published (closes #2157366)