Fix avatar sync for a SSO

[klaufel]

Working on the web "http://nintenderos.com" we were having an avatars synchronization problem. Reading the official documentation of Disqus I observed that the value of the parameter that is required in the configuration "avatar" is a url and the function that currently exists get_avatar() returns <img src="..."> when it should return a string with a url.

Image with Disqus documentation:

I have modified the function by get_avatar_url() so that it always returns the url of the avatar, so when the avatar value is updated it will show the new avatar.

[webrender]

Thanks for this PR! We're going to run some tests against it and hopefully release an update which includes it tomorrow.

[dmatt]

@klaufel We finished testing and your PR made it into the recent 3.0.16 release, https://wordpress.org/plugins/disqus-comment-system/#developers. Thank you!

[psbaj]

Thank you so much for this fix! I contacted disqus support last year about this issue and they told me that the problem was my server ¬¬ well, not a problem anymore :)