The ArrayBuffer holding the data might be over-sized in case the
data length was not known during the transfer, e.g. when using a
Content-Encoding other than `identity` or when using a
Transfer-Encoding.
Only the view into the buffer has the correct length then, hence
always use the view directly when creating the blob URI for the
download, instead of the over-sized underlying buffer.
Closes GH-3627
Closes GH-3634
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