Abstract `_request()` to a common utility so we can use inside a filter
`wp core update` should use cached files too
Copy cache to a temp file, so upgrader can later delete it
Subclass, because the filter we want to use isn't available until 3.7.1
Only indicate "Downloading" when we're actually downloading something
Make this method generally more useful
Validate methods
Allow `\Requests` to validate method on its own
Pass args in expected order, and allow `$data` as an option too
Merge pull request #1320 from wp-cli/cache-update

Core updates should be stored in the cache directory