Motherless extractor: fix gallery media metadata issues

[seanbowve]

Caution

Links in this PR are NSFW by nature of the website and surrounding material, though I've tried to use a less NSFW URL as an example.

The current extractor has two metadata issues when used through a gallery:

1. Metadata isn't retrieved for photos
2. Any metadata retrieved for the file is clobbered by the gallery's metadata, most notably uploader

I don't think file.update(data) is the way to go here, but I also noticed both of these problems were somewhat addressed for the group extractor anyway, so I copied that logic in the gallery extractor.

The example is run on a gallery by seanbowve, containing one photo that was uploaded by pepsibackbaby.

Before

$ gallery-dl -K https://motherless.com/G8978F51
Keywords for --chapter-filter:
------------------------------
category
  motherless
subcategory
  gallery

Keywords for directory names:
-----------------------------
category
  motherless
count
  1
gallery_id
  8978F51
gallery_title
  ##Repro Gallery##
id
  26AF63A
num
  1
subcategory
  gallery
thumbnail
  https://cdn5-thumbs.motherlessmedia.com/thumbs/26AF63A.jpg
title
  Avril Lavigne/sexy/as/hell/legs
type
  image
uploader
  seanbowve # <=== incorrect, should be pepsibackbaby
url
  https://cdn5-images.motherlessmedia.com/images/26AF63A.jpg

Keywords for filenames and --filter:
------------------------------------
category
  motherless
count
  1
extension
  jpg
filename
  26AF63A
gallery_id
  8978F51
gallery_title
  ##Repro Gallery##
id
  26AF63A
num
  1
subcategory
  gallery
thumbnail
  https://cdn5-thumbs.motherlessmedia.com/thumbs/26AF63A.jpg
title
  Avril Lavigne/sexy/as/hell/legs
type
  image
uploader
  seanbowve # <=== incorrect, should be pepsibackbaby
url
  https://cdn5-images.motherlessmedia.com/images/26AF63A.jpg

After

$ gallery-dl -K https://motherless.com/G8978F51
Keywords for --chapter-filter:
------------------------------
category
  motherless
subcategory
  gallery

Keywords for directory names:
-----------------------------
category
  motherless
count
  1
date
  2013-03-13 00:00:00
favorites
  2
gallery_creator
  seanbowve
gallery_id
  8978F51
gallery_title
  ##Repro Gallery##
group
  
id
  26AF63A
num
  1
subcategory
  gallery
tags[N]
  0 avril
  1 Lavigne/sexy/as/hell/legs
  2 boobs
thumbnail
  https://cdn5-thumbs.motherlessmedia.com/thumbs/26AF63A.jpg
title
  Avril Lavigne/sexy/as/hell/legs
type
  image
uploader
  pepsibackbaby
url
  https://cdn5-images.motherlessmedia.com/images/26AF63A.jpg
views
  101

Keywords for filenames and --filter:
------------------------------------
category
  motherless
count
  1
date
  2013-03-13 00:00:00
extension
  jpg
favorites
  2
filename
  26AF63A
gallery_creator
  seanbowve
gallery_id
  8978F51
gallery_title
  ##Repro Gallery##
group
  
id
  26AF63A
num
  1
subcategory
  gallery
tags[N]
  0 avril
  1 Lavigne/sexy/as/hell/legs
  2 boobs
thumbnail
  https://cdn5-thumbs.motherlessmedia.com/thumbs/26AF63A.jpg
title
  Avril Lavigne/sexy/as/hell/legs
type
  image
uploader
  pepsibackbaby
url
  https://cdn5-images.motherlessmedia.com/images/26AF63A.jpg
views
  101