Make column counter variable available in filter

This commit simply makes the existing column counter variable (`$i`) available as a parameter within the `edd_download_class` filter hook. This is beneficial in cases where you want to apply specific styles to downloads in the grid based on which column they are in or their order in the list.

Example usage:

```php
function my_edd_download_class( $class, $id, $atts, $i ) {
	if ( 0 === $i % 2 ) {
		$class .= ' edd_download_even';
	}
	return $class;
}
add_filter( 'edd_download_class', 'my_edd_download_class', 10, 4 );
```
Merge pull request #2339 from coreymckrill/patch-2

Make column counter variable available in filter
Fix Error 400 Bad Request on API call

I was running into the same issue as mentioned at https://easydigitaldownloads.com/support/topic/error-400-bad-request-on-api-call/ - license key activation does not work if you include the `$params` in the request body. You get a 400 bad request. By removing this line, the license activation is successful.
Merge pull request #2340 from davekiss/patch-1

Fix Error 400 Bad Request on API call
Merge branch 'master' into release/2.0