decompile not producing correct output in methods with optional params

Consider the example:

``` fsharp
type Foo =
    static member Foo(?x : int) = ()

decompile <@ Foo.Foo() @>              // "Foo.Foo(None)" 
decompile <@ Foo.Foo(x = 42) @>        // "Foo.Foo(Some(42))"
decompile <@ Foo.Foo(?x = Some 42) @>  // "Foo.Foo(Some(42))"
```

This does not produce valid F# in any of the cases.
