Add release-date of an episode to tv-grabber ttvdb4.py

The 'aired' field retrieved by the TV grabber for thetvdb.com API v4
for a specific episode seems to be compatible to the 'releasedate'
item of the xml format required by MythTV documented at
https://www.mythtv.org/wiki/MythTV_Universal_Metadata_Format

Refs #628

(cherry picked from commit c11c85a)

mythtv/bindings/python/ttvdbv4/myth4ttvdbv4.py

@@ -406,6 +406,10 @@ def _format_xml(self, ser_x, sea_x=None, epi_x=None):
             m.season = check_item(m, ("season", epi_x.seasonNumber), ignore=False)
             m.episode = check_item(m, ("episode", epi_x.number), ignore=False)
             m.runtime = check_item(m, ("runtime", epi_x.runtime), ignore=True)
+            if epi_x.aired:
+                # convert string to 'date' object, assuming ISO naming 
+                m.releasedate = convert_date(epi_x.aired[:10])
+
         desc = strip_tags(desc).replace("\r\n", "").replace("\n", "")
         m.description = check_item(m, ("description", desc))