Core: use uppercase hexadecimal digits for percent-encoding.

RFC3986 says that, for consistency, URI producers and normalizers
should use uppercase hexadecimal digits for all percent-encodings.

This is also what modern web browsers and other tools use.

Using lowercase hexadecimal digits makes it harder to interact with
those tools in case when use of the percent-encoded URI is required,
for example when $request_uri is part of the cache key.

Signed-off-by: Piotr Sikora <piotr@cloudflare.com>