Backport WP 4.4.1 fixes.

[joemcgill]

This is a placeholder ticket to backport several bugfixes from the WordPress 4.4.1 release to the plugin – namely:

• Always include the src in the srcset. 36150
• Add compatibility for versions < 2.7 when the full image path was stored in the metadata36151
• Fix the check whether the attachment meta matches the image src to work with http/https and CDNs 36152

Also, keep an eye on 34945, which might make it into 4.4.1 in the next few days.

[joemcgill]

Now that this has hung around so long, there are other changes that will need to be accounted for as well as those listed above. We need to think about prepping for 4.5 as well.