Fixed try_files directory test to match only a directory.

Historically, it was possible to match either a file or directory
in the following configuration:

    location / {
        try_files $uri/ =404;
    }

src/http/ngx_http_core_module.c

@@ -1353,7 +1353,7 @@ ngx_http_core_try_files_phase(ngx_http_request_t *r,
             continue;
         }
 
-        if (of.is_dir && !test_dir) {
+        if (of.is_dir != test_dir) {
             continue;
         }