[bugfix] Correctly handle range > content-length

[Jadeiin]

Description

Hi I found this bug in Pleroma/Akkoma's MediaProxy. Currently GTS instance returns incorrect range headers when suffix length is larger than content-length:

$ url="https://goblin.technology/fileserver/01BPSX2MKCRVMD4YN4D71G9CP5/attachment/original/01E4BA3HFHWN7GDE2Y89F0J563.png"
$ curl -r 0-1048576 -X GET -I $url | grep Content-Range
Content-Range: bytes 0-1048576/11021

The correct range header should be bytes 0-11020/11021, which only requires formatting correct endRng when range > content-length.

Related issue(closed but not actually fixed): #1978

Checklist

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[tsmethurst]

Oh nice, thank you :)