HTTP: fixed r.headersOut setter for special headers.

HTTP: fixed r.headersOut setter for special headers.

The issue was introduced in 5b7676ec600d (0.7.5) when njs module was
adapted to changes in nginx/1.23 related to header structures.

When special headers (Content-Length, Content-Type, Content-Encoding)
were set, the value of the last outgoing header might be overwritten
with a new set value.

This closes #555 issue on Github.