check_basket_is_valid recalculates purchase info of a basket Line

[mblayman]

I work on an application where price calculation is dynamic and expense computationally. While trying to optimize my application code, I found that check_basket_is_valid does not use the cached purchase_info property available for basket lines. Because of this, I have certain Django views that are doing redundant work.

Since it calls fetch_for_line directly, it bypasses the proxying work and cache that Line.purchase_info provides.

[mvantellingen]

Can you supply a PR for this? Thanks :-)

[sasha0]

This won't work since availability and stockrecord will be cached, and then, if we allocate remaining number of the stock record and it becomes unavailable - check_basket_is_valid would still pass.

I added tests to verify this.

[sasha0]

Closing since my test proofs appropriateness of current implementation. If you have other suggestion for performance improvement - please open a new issue. Thanks!