Having a base abstract migration breaks loading metadata from code first context

[kamfel]

Hi!

I tried to incorporate this library in my tests but I got an exception: System.MissingMethodException: 'Cannot create an abstract class.'.
The problem seems to be in CodeFirstMetadataWorkspaceFactory.GetCacheName(), where the method tries to instantiate the base abstract migration.

Steps to reproduce:

1. Have an abstract migration in the same project as your DbContext
2. Try to load metadata
   var metadata = ModelMetadataGenerator.LoadFromCodeFirstContext(str => new AppDbContext(str), true);