Enhancement: resolve shortcodes to retrieve og images

The OpenGraph image function looks at the post_content before any filters are applied. This may mean that there are shortcodes present which would add images (think image galleries), but as they have not been resolved yet, those images are not added as og:image meta tags.

This change enhances the content by resolving the shortcodes before searching for the images.
The fix *does* not apply all filters, not does it change the global state of post_content.

Change made based on suggestion in [this forum thread](http://wordpress.org/support/topic/ogimage-set-to-default-image-but-articlepage-has-own-images).