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>

src/core/ngx_string.c

diff --git a/src/core/ngx_string.c b/src/core/ngx_string.c
index 4e27917..9d854fa 100644
--- a/src/core/ngx_string.c
+++ b/src/core/ngx_string.c
@@ -1407,7 +1407,7 @@
 {
     ngx_uint_t      n;
     uint32_t       *escape;
-    static u_char   hex[] = "0123456789abcdef";
+    static u_char   hex[] = "0123456789ABCDEF";
 
                     /* " ", "#", "%", "?", %00-%1F, %7F-%FF */