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.

prioritynormal
typebug
done in1.9.3
load0.100
load left0.000
closed by#5ad30b94a083 [entities] BlogEntry.dc_date() should return publication date when published (closes #2157366)