Fixes regarding external assembly references which cannot be resolved

[mjgertsen]

When saving an assemblybuilder to file I had two issues. First one was a reference which could not be found when a type of a references assembly was passed as a parameter of an customattribute. Second one that I had was that the IsReferencedType method was always returning false. Therefore I got an exception that the referenced assembly (the one that was being saved) could not be found.