Minor fixes to Facebook widget plugin for SSL

Facebook forces SSL so linking the iframe to http:// causes an unnecessary redirect to https://
This line could probably be better written as `set_url_scheme( $like_args['href'], 'https' );` I think?  This current str_replace also would catch a http later in the url -- for example a query arg containing a url, if not using `set_url_scheme()` we should use `preg_replace( '#^(http:)?//#i', 'https://', $like_args['href'] )` to ensure we're only swapping out the raw protocol, that we handle relative protocols, and do it case insensitively -- `str_replace()` would fail on `HTTP://` and the like (legacy code was at fault for this as well).

But I'd prefer to keep it tidy with `set_url_scheme()`