radio: don't get stuck on mpd http streams
radio: don't get stuck on mpd http streams

MPD HTTP streaming provide the media content at every URL thrown at the
HTTP backend. So requests for shoutcast and icecast metadata get stuck
receiving the actual media content, instead of the expected metadata.

The conclusion is to only request these metadata files, when they're not
actually advertised as audio or video content types in a HEAD request.

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