Make decode_uri the inverse of encode_uri (#2540)

decode_uri was a byte-for-byte copy of decode_uri_component: it decoded every
%XX, including escapes of the reserved characters that encode_uri leaves
literal. So decode_uri was not the inverse of encode_uri and promoted an
escaped delimiter into a real one -- decode_uri("http://h/a%2Fb") returned
"http://h/a/b". Keep escapes of the reserved set encode_uri preserves, matching
JS decodeURI; non-reserved escapes still decode.

httplib.h

@@ -10603,7 +10603,19 @@ inline std::string decode_uri(const std::string &value) {
     if (value[i] == '%' && i + 2 < value.size()) {
       auto val = 0;
       if (detail::from_hex_to_i(value, i + 1, 2, val)) {
-        result += static_cast<char>(val);
+        auto c = static_cast<char>(val);
+        // Keep escapes of the reserved characters that encode_uri leaves
+        // literal, so decode_uri is the inverse of encode_uri and an escaped
+        // delimiter is not promoted into a real one (as with JS decodeURI).
+        if (c == ';' || c == '/' || c == '?' || c == ':' || c == '@' ||
+            c == '&' || c == '=' || c == '+' || c == '$' || c == ',' ||
+            c == '#') {
+          result += value[i];
+          result += value[i + 1];
+          result += value[i + 2];
+        } else {
+          result += c;
+        }
         i += 2;
       } else {
         result += value[i];