Add ability to clear cookies from a CookieStore

[pfernie]

Addresses #613 for a simple "batteries included" quality-of-life improvement.

Adds method clear(&self) to trait cookies::CookieStore, allowing
users to clear a store's contents.

Adds method clear_cookies(&self) to Client under feature cookies.

Alternatively, instead of adding this to the cookies::CookieStore trait and expanding the Client API, the cookies::Jar impl could be extended with the requested method directly. However, this does not seem to be as "batteries included" for users that simply want to utilize the default Jar via ClientBuilder::cookie_store(true).

[Okabe-Rintarou-0]

Is this issue still open? I also need to clear cookies

[pfernie]

The original change here is breaking, as it has been labelled (and I did not keep on top of for inclusion in last breaking release...). I've rebased the existing PR for current master, but the change will not be eligible for merging until the next breaking release of reqwest.

In the meantime, you can use something like reqwest_cookie_store which gives you more direct access to the underlying store, which does expose a clear() method.