Writing cookies: Prefix wildcard domains with "."

If a cookie matches a wildcard domain, e.g. "(anything).example.com",
then the line written out to a Netscape-formatted cookies.txt file must
begin with a dot, e.g. ".example.com" not "example.com".  This is
redundant with the second field of the line, a TRUE/FALSE value
indicating whether the domain is a wildcard, but that's the way it goes.
Add test for cookies that match subdomains

A test to go with commit dae2733: Tests that when writing a cookies.txt
file:

* Cookies that only match exactly the domain specified must not have a
  leading dot, and must have FALSE as the second field.
* Cookies that match subdomains must have a leading dot, and must have
  TRUE as the second field.
Merge pull request #295 from mmorearty/patch-cookiedomain

Writing cookies: Prefix wildcard domains with "."