proxying generic interface with open generic type parameters throws vague ArgumentException

[blairconrad]

Found while writing characterization tests before starting on #34 proper.

Consider this test:

[Test]
public void Cannot_proxy_generic_type_with_open_generic_type_parameter()
{
    var innerType = typeof(IList<>);
    var targetType = innerType.MakeGenericType(typeof(IList<>));
    generator.CreateInterfaceProxyWithoutTarget(targetType, new IInterceptor[0]);
}

which corresponds to trying to proxy an IList<IList<>>.

It fails with

Test 'Castle.DynamicProxy.Tests.BasicInterfaceProxyTestCase.Cannot_proxy_generic_type_with_open_generic_type_parameter' failed:
    System.ArgumentException : Cannot build type parameter for custom attribute with a type that does not support the AssemblyQualifiedName property. The type instance supplied was of type 'System.RuntimeType'.

Compare this to the attempt to make proxy an IList<>, which returns an ArgumentException with the message

You can't specify a generic type definition.
Parameter name: interfaceToProxy

This is not a big deal for FakeItEasy, I don't think, and probably less of a problem overall than #34, but if there's interest, I can look into it alongside or after the fix for #34.