handler: default to unmodified field name, as encoding/json

When assigning positional names, use the same rule that encoding/json does for
unlabelled fields, viz., the field-name as spelled.  The encoder already
handles case-insensitive lookup.

handler/positional.go

@@ -61,8 +61,7 @@ func structFieldNames(atype reflect.Type) (bool, []string) {
 			// handled by the cases above.
 			continue
 		}
-		name := strings.ToLower(fi.Name[:1]) + fi.Name[1:]
-		names = append(names, name)
+		names = append(names, fi.Name)
 	}
 	return true, names
 }