Deal with 200 (rather than 206) response to get-range request

[edsko]

I saw this happening but I do not remember the circumstances and cannot reconstruct it. If it happens again, we can deal with it once we understand better why it's happening (perhaps the right thing to do is to change the ServerCapabilities to record that this server does not support range requests after all, even though we concluded it did). Downloading the entire file is not the right thing to do: we don't want to download the uncompressed index.

For now a 200 response to a get-range request will result in an exception.