I was unable to find any clear guidance in RFC-2616 or elsewhere.
It looks like they simply forgot to mention if token called
"bytes-unit" is case-sensitive or not. Generally fields are
not sensitive, but there are exceptions. Certainly, it is valid
to accept extra units, if we deem "bytes=" and "BYTES=" different.
This patch came about because OpenStack Swift throws an exception
on WebOb 1.2. It worked in WebOb 1.0.8, where we had a code that
invoked .lower() before matching.