REST API: Fix Missing videopress_guid in upload media response

Summary: During testing of the new Media Video Widget, the videopress_guid which is needed to construct the proper oembed url was missing in API responses for me.  The following minor change fixes this issue.  Additional details #7215

Test Plan: See the PR linked above for test instructions

Reviewers: codebykat

Reviewed By: codebykat

Subscribers: dbtlr

Differential Revision: https://[private link]

Merges r156992-wpcom.

json-endpoints/class.wpcom-json-api-upload-media-v1-1-endpoint.php

@@ -57,7 +57,7 @@ function callback( $path = '', $blog_id = 0 ) {
 		        // More than likely a post has not been created yet, so we pass in the media item we
 		        // got back from the Jetpack site.
 		        $post = (object) $media_item['post'];
-		        $media_item = $this->get_media_item_v1_1( $media_item['ID'], $post, $media_item['file'] );
+		        $media_item = $this->get_media_item_v1_1( $post->ID, $post, $media_item['file'] );
 	        }
         }