Manually passed `Cookie` header overrides `http.CookieJar` cookies

[yauheni-chaburanau-it]

Description
If you manually pass Cookie header in DialContext(..., http.Header), cookies from Dialer.Jar will be overwritten.

Steps to Reproduce

dialer := websocket.Dialer{
	Jar: jar,
}

header := http.Header{}
header.Set("Cookie", "some_cookie_name=some_cookie_value")

... = dialer.DialContext(ctx, url, header)

Possible reason
From the first look I would say that this is happening because the part of code which is responsible for setting up all the passed headers ignores already applied Cookie header from http.CookieJar.