When reading files from B2 via cloudflare using --b2-download-url
cloudflare strips the Content-Length headers (presumably so it can
inject stuff into the body).
This caused rclone to think the file was corrupted as the length
didn't match.
The patch uses the old length read from the listing if there is no
Content-Length.
box: fix transfers getting stuck on token expiry after API change
Box recently changed their API, changing the case of returned API items
> On May 10th, 2021, as part of our continued infrastructure upgrade,
> Box's API response headers will standardize to return in a case
> insensitive manner, in line with industry best practices and our API
> documentation. Applications that are using these headers, such as
> "location" and "retry-after", will need to verify that their
> applications are checking for these headers in a case-insensitive
> fashion.
Rclone was reading the raw headers from the `http.Header` and not
using the `Get` accessor method which meant that it was sensitive to
case changes.
This fixes the problem by using the `Get` accessor method.
