[extractor/imgur] Fix Imgur Extractor

[trwstin]

Fix Imgur Extractor Downloading Some Videos Incorrectly

This PR adds an 'Accept' header with the value '/' to the request sent to Imgur servers when downloading videos. It is done by modifying ImgurIE to include 'http_headers': {'Accept': '*/*'} under the return statement of _real_extract function.

The change resolves issues with downloading certain video formats from Imgur URLs or those embedded in Reddit posts, ensuring that the download does not result in a corrupted file.

Fixes #9458

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