imgur: download videos as mp4 instead of gif

Some imgur URLS have the extension ".gifv" and show up as a gif,
even though they're actually supposed to be mp4 videos. Imgur
serves all videos/gifs as both .gif and .mp4. The image dict has
a key "prefer_video" to distinguish the two. This commit
overrides the .gif extension if "prefer_video" is true to ensure
we download the submission as originally intended.

bdfr/site_downloaders/imgur.py

@@ -32,7 +32,11 @@ class Imgur(BaseDownloader):
         return out
 
     def _compute_image_url(self, image: dict) -> Resource:
-        image_url = 'https://i.imgur.com/' + image['hash'] + self._validate_extension(image['ext'])
+        ext = self._validate_extension(image['ext'])
+        if image.get('prefer_video', False):
+            ext = '.mp4'
+
+        image_url = 'https://i.imgur.com/' + image['hash'] + ext
         return Resource(self.post, image_url, Resource.retry_download(image_url))
 
     @staticmethod