"Extract Interface" refactoring doesn't offer to add static members to the interface

Invoke "Extract interface" refactoring on this class
```
class TestExtractInterface
{
    public void M1() { }
    public static void M2() { }
}
```

Observed:
There is no offer to include M2 into the interface, only M1 is in the list.

Expected:
Should offer M2 as well. Should decide if it should be included by default.
