Variables in the debugger are not displayed if type is PSObject

[rkeithhill]

I see this when debubbing a filter definition like:

filter foo {
    "`$_ is $_"
}

In the Variables window $_ shows up with no value. I believe the problem is that the code that checks to see if the variable is expandable returns true on primitives wrapped in a PSObject. I have a fix I'll submit in a few.