SyntaxGenerator.AsPublicInterfaceImplementation doesn't set the accessibility to public in VB

Repro:

``` csharp
SyntaxGenerator generator = SyntaxGenerator.GetGenerator(document);
var method = generator.MethodDeclaration("Test");
method = generator.AsPublicInterfaceImplementation(method, generator.IdentifierName("IDisposable"));
var acc = generator.GetAccessibility(method);
```

In VB, the method gets the Implements clause but is not made public. In C#, the method is made public.
