Site accelerator/Photon processes Gravatars w/ already processed image URLs (resulting in broken images)

## The Issue

I am filtering the `default` parameter for Gravatars with an image URL I obtain via `wp_get_attachment_image_src`. (Photon processes this URL).

The URL is processed a 2nd time as Photon filters the Gravatar output. The result is a broken URL that looks like:

`https://secure.gravatar.com/avatar/e72425f2a568bcfb144b752a793c1384?s=48&d=https%3A%2F%2Fi2.wp.com%2Fsixprizes.com%2Fwp-content%2Fuploads%2F2017%2F05%2Ftwitter.png%3Ffit%3D512%252C512%26ssl%3D1&r=pg`

which redirects to:

`https://i1.wp.com/i2.wp.com/sixprizes.com/wp-content/uploads/2017/05/twitter.png?ssl=1` (which is invalid)

Normally, Jetpack will bail on processing the URL when it has `i2.wp.com` as its host. (See `validate_image_url` in `class.photon.php`.) However, it doesn't pick up on this when it's parameterized in a Gravatar URL.

I am working around the issue by removing the `jetpack_photon_override_image_downsize` filter before getting the image with `wp_get_attachment_image_src`.

## Function

```php
add_filter( 'pre_get_avatar_data', 'ac_pre_get_avatar_data_default_url', 10, 2 );

function ac_pre_get_avatar_data_default_url( $args, $id_or_email ) {

// Photon: Disable
add_filter( 'jetpack_photon_override_image_downsize', '__return_true' );

// Get image
$image_src = wp_get_attachment_image_src( 65904, 'full' );

// Photon: Reenable
remove_filter( 'jetpack_photon_override_image_downsize', '__return_true' );

// Image URL
$image_url = $image_src[0];

// Set default
$args['default'] = $image_url;

// Return
return $args;
}
```

## URL when Photon is disabled

`https://secure.gravatar.com/avatar/e72425f2a568bcfb144b752a793c1384?s=96&d=https%3A%2F%2Fsixprizes.com%2Fwp-content%2Fuploads%2F2017%2F05%2Ftwitter.png&r=pg`

which redirects to:

`https://i1.wp.com/sixprizes.com/wp-content/uploads/2017/05/twitter.png?ssl=1` (which is valid)