bugfix: remove null terminator

[n-bes]

Without this patch I got:

> curl: (8) Nul byte in header

location = /bye {
    content_by_lua_block {
        local resp = require "ngx.resp"
        resp.set_status(500, "custom status")
        ngx.say("set_status_and_reason")
    }
}

[n-bes]

Introduced by 3416740