Bridge fails to download media that requires authentication.

[Techwizz-somboo]

When a user is on a different homeserver than the bot, eg matrix.org vs matrix.myserver.org, media will fail to download.

[Techwizz-somboo]

This should be fixed by 0a86c05.

[Techwizz-somboo]

I'll reopen this until my PR is merged, and I integrate with MatrixClient downloadAuthenticatedContent() instead. turt2live/matrix-bot-sdk#375