config.For<T>(t=>t.Property) should not use default if setting configured

Description

config.For<ArticlePage>(t => t.MainBody)
    .AddPlugin("print");
 
// further down in the code (or in another module)
config.For<ArticlePage>(t => t.MainBody) // This will now be based on Default and not the previous config
    .AddPlugin("cut");

Expected:
The second time you do a For<>, it should continue on the settings object. It should now have
the print and cut plugin

Actual:
It resets to Default then adds the cut plugin.