Send the full comment object to `get_avatar()` rather than just the comment author's email address

`get_avatar()` can accept a user ID, email address, or comment object.  It's best to use a comment object because it provides the most context to plugins which use the `get_avatar` filter to change the response of the `get_avatar()` function.

Note that core WordPress always sends the full comment object to `get_avatar()` when rendering comment avatars.

Jetpack uses the `get_avatar` filter to replace the default avatar with the Twitter or Facebook avatar for comments submitted using Twitter or Facebook authentication.  Jetpack needs the full comment object to do this.