oembed_cache created in post affects the post_author on export

[wecanwang]

To fix ticket #34115 we've introduced a custom type oembed_cache in the post table. However, once an oembed_cache is created the post_author will be the user who triggers it instead of the site owner. Because of the reason above, when you try to export the blog, it'll show up as a <wp:author> in the file which is pretty confusing. It probably makes more sense to set the post_author as 0 or set it to the site owner.