Custom Cache Keys Must Use Uppercase HEX Characters

After debugging an invalid argument error with Fastly support we realized that custom cache keys must be 64 uppercase HEX characters and right now they are lowercase HEX characters.

https://github.com/fastly/js-compute-runtime/pull/281/files#diff-bca0f8f03afc68c49c0881002ded55236dc27932e34e1f60a7827a9bbe54c58fR92