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>