(Frosting) Support registering open generics

[asalvo]

The Cake Frosting container registration does not support open generics. It throws an exception because the call to IsAssignableFrom fails on line 22 of RegistrationBuilder.

Open generic example:
services.RegisterType(typeof(Logger<>)).As(typeof(ILogger<>)).Singleton();

I have a simple work around at the moment where I just register everything explicitly as there is not that many registrations to do.

There are several examples online of code which claims to create an IsAssignableFrom that supports open generics. Would you accept a pull request that adds support for this?

[devlead]

Sure we would review such a PR.

[gep13]

@asalvo was this something that you were interested in contributing?