Improved $cookie_ evaluation.

Improved $cookie_ evaluation.

In case "Cookie" header is sent by client, multiple cookie pairs were
incorrectly split by a semicolon and comma.

Now they are split by a semicolon only.

For example, next variables will be found for "Cookie: a=b, c=d; e=f":
- $cookie_a: "b, c=d"
- $cookie_e: "f"

Closes #1042 on GitHub.