David Garcia

This replacement fixes mixed content warning that occurs when moving a blog to https without replacing current http style links.

Double Slash Domains

This plugin removes the http/https schema from urls inside the blog source code and replace them with double slash.

Stefan Kittel

It easily solves the problem of mixed unsafe content.
Once a web page has its contents, such as CSS, JavaScript or graphics loads unencrypted, the browser reports the whole website as unsafe.
This message will appear with this or similar texts “Connection is not secure” and “Parts of this page are not secure. This can e.g. be graphics. “.
Now you can check and correct all posts, comments, plugins and the theme …
Or you simply install “SSL Fixit”. 
Finished. There is nothing to configure.
The plugin simply replaces all http: // in the output with https: //.
However, if you still need unencrypted links, you can not use this plugin. 
But that should / should be there in the meantime only in exceptional cases.

SSL Fixit Plugin for WordPress by Stefan Kittel

A simple plugin to fix the html unsecure mixed content error

Double Slash Domains vs SSL Fixit Plugin for WordPress by Stefan Kittel