`WP_oEmbed_Controller::get_proxy_item()` should remove `_wpnonce` from cached `$args`

[r-a-y]

When caching oEmbed proxy requests:
​https://github.com/WordPress/WordPress/blob/4.8-branch/wp-includes/class-wp-oembed-controller.php#L159

The following arguments are used to generate the cache key:

Array
(
    [format] => json
    [maxwidth] => 600
    [discover] => 
    [url] => URL
    [_wpnonce] => 94f457a20b
)

_wpnonce is added because of wp.media.view.settings.nonce.wpRestApi.

Since _wpnonce is variable, I think it would make sense to remove the _wpnonce key from the $args array so the cache is more likely to be hit.

It also makes it easier for plugins to reference the oEmbed transient when overriding wp_media_shortcode() for use with the Video Widget.

[r-a-y]

Patched against 4.8 branch

[westonruter]

The nonce removal is also included on the patch in #41299.

[westonruter]

In 41035:

REST API: Remove _wpnonce value from being used in hashed oEmbed proxy cache key.

Amends [40628].
Props r-a-y, westonruter.
See #40450.
Fixes #41048.

[westonruter]

Re-opening for 4.8.1.

[westonruter]

In 41048:

REST API: Remove _wpnonce value from being used in hashed oEmbed proxy cache key.

Merges [41035] to 4.8 branch.
Amends [40628].
Props r-a-y, westonruter.
See #40450.
Fixes #41048 for 4.8.1.