Bugfix: do not discard cookie default policy from jar

[mikran]

Summary

http.cookiejar allows setting a custom default policy for cookies. httpx immediately discards that before any requests get sent

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.