use the more correct mask

src/http/ngx_http_parse.c

diff --git a/src/http/ngx_http_parse.c b/src/http/ngx_http_parse.c
index b83810d..2e38434 100644
--- a/src/http/ngx_http_parse.c
+++ b/src/http/ngx_http_parse.c
@@ -775,7 +775,7 @@
             if (c) {
                 hash = ngx_hash(hash, c);
                 r->lowcase_header[i++] = c;
-                i &= ~NGX_HTTP_LC_HEADER_LEN;
+                i &= (NGX_HTTP_LC_HEADER_LEN - 1);
                 break;
             }