While writing the GOPROXY docs it occurred to me that versions
can contain upper-case letters as well. The docs therefore say
that versions are case-encoded the same as paths in the proxy
protocol (and therefore in the cache as well). Make it so.

Change-Id: Ibc0c4af0192a4af251e5dd6f2d36cda7e529099a
Reviewed-on: https://go-review.googlesource.com/124795Reviewed-by: Bryan C. Mills <bcmills@google.com>