Includes:

• closes .length no longer works #457
• issue test case

First time contributing, my fix is to remove the following if check in order to return a normal ObjectDescriptor within the ObjectDescriptorProvider.TryGetDescriptor(...) method.

// ObjectDescriptorProvider.cs
public bool TryGetDescriptor(Type type, out ObjectDescriptor value)
{
    if (type == StringType)
    {
        value = ObjectDescriptor.Empty;
        return false;
    }
    ...
}

I don't fully understand the reasoning behind this if check.
This if check was added in "Introduce reactive helper registration, improve general performance" - commit: 9d52004.

Anyway this PR removes this check, let me know if you don't think this is a good change or have any suggestions.