[Instagram] Allow Sidecar posts taken_at to populate date

[bradenhilton]

Recently I noticed that Sidecar post images weren't sorting by mtime in a way I'd expect. Evidently it's because they do not return a date value in their metadata:

❯ gallery-dl --cookies <cookies.txt> -j -- "https://www.instagram.com/p/BoHk1haB5tM/"
[
  ...
  [
    3,
    "https://scontent-lcy1-1.cdninstagram.com/v/t51.2885-15/40758827_2138611023072230_4073975203662780931_n.jpg?stp=dst-jpg_e35&_nc_ht=scontent-lcy1-1.cdninstagram.com&_nc_cat=102&_nc_ohc=XGO4XoxbCekAX_UqD53&edm=ALQROFkBAAAA&ccb=7-5&ig_cache_key=MTg3NTYyODgzNzQxNTI3MDM0NQ%3D%3D.2-ccb7-5&oh=00_AT_PAS7ojN9bi5FJY677ochh-llalkMjr_OtGnrogyukCw&oe=62A30A48&_nc_sid=30a2ef",
    {
      ...
      "date": null,
      ...
    }
  ],
  ...
]

I believe this line of code

gallery-dl/gallery_dl/extractor/instagram.py

Line 331 in c2d1171

media = image

destroys all of the metadata that media gets from post["media"], including post["media"]["taken_at"].

If I change

gallery-dl/gallery_dl/extractor/instagram.py

Lines 335 to 336 in c2d1171

	"date" : text.parse_timestamp(item.get("taken_at") or
	media.get("taken_at")),

to

 "date"       : text.parse_timestamp(item.get("taken_at") or
-                                    media.get("taken_at")),
+                                    media.get("taken_at") or
+                                    post.get("media", {}).get("taken_at")), 

the date metadata is returned successfully:

❯ gallery-dl --cookies <cookies.txt> -j -- "https://www.instagram.com/p/BoHk1haB5tM/"
[
  ...
  [
    3,
    "https://scontent-lcy1-1.cdninstagram.com/v/t51.2885-15/40758827_2138611023072230_4073975203662780931_n.jpg?stp=dst-jpg_e35&_nc_ht=scontent-lcy1-1.cdninstagram.com&_nc_cat=102&_nc_ohc=XGO4XoxbCekAX_UqD53&edm=ALQROFkBAAAA&ccb=7-5&ig_cache_key=MTg3NTYyODgzNzQxNTI3MDM0NQ%3D%3D.2-ccb7-5&oh=00_AT_PAS7ojN9bi5FJY677ochh-llalkMjr_OtGnrogyukCw&oe=62A30A48&_nc_sid=30a2ef",
    {
      ...
      "date": "2018-09-24 18:08:34",
      ...
    }
  ],
  ...
]

Put simply, I'd like to be able to use post["media"]["taken_at"] to populate date metadata on each file.

Note: This isn't a PR as it's not particularly elegant and I'm unsure if it contains any breaking changes. If it does, perhaps this could be achieved with an option/config entry?