Update tests for fetching source data from Pixiv.

* Use a different Pixiv post because the old one has
  been deleted from Pixiv.
* Test that the original tag is added when the Pixiv
  post is in the Original category on Pixiv.
* Update the VCR cassettes.
Refactor fetching the metadata from the API.

The functionality is unchanged, this is just a code cleanup.
Avoid unneeded HTTP request when rewriting URLs.

Calling `source.get` would potentially trigger two HTTP requests to
Pixiv to gather all the information it needs. It needs to make a
request to the HTML page to get the Original flag, and possibly a
request to the API to get the image directory.

So instead we use `source.get_image_url`, which only needs to make one
request to the API to gather all the information it needs to build the
URL.
Check for original works on Pixiv.

Sometimes on Pixiv an illustration is marked as an original work, but
it doesn't have an original tag. Scrape the HTML page and add an
original tag for this case.