Connection avatar downgraded to HTTP

[ChaosExAnima]

This is related to #17 and #18. The code to display connection avatars uses set_url_scheme, which changes the scheme based off of the current site configuration. However, in many cases that would mean that the avatar is downgraded to HTTP when HTTPS is available, or even in the case of Google's avatars where a non-SSL version is not available.

I would propose instead that the following logic:

• If the avatar is HTTPS, keep the URL unchanged.
• If the avatar is HTTP, add a filter or other method to allow services to declare SSL should be forced. If this is set to true, set the schema to HTTPS.
• If the service did not opt in, use the current logic.

Happy to contribute a PR if desired.