Cmdlet default parameter set is all parameter sets

Fixed loading and executing a cmdlet that defines its default parameter set to be the AllParameterSet:

```C#
    [Cmdlet(VerbsDiagnostic.Test, "MyCmdlet",
        DefaultParameterSetName = ParameterAttribute.AllParameterSets)]
    public sealed class MyCmdlet: PSCmdlet
    {
        [Parameter(Position = 0)]
        [ValidateNotNullOrEmpty]
        public string First { get; set; }

        [Parameter(Position = 1, ParameterSetName = "Set1")]
        [Parameter(Position = 1, ParameterSetName = "Set2")]
        [ValidateNotNullOrEmpty]
        public string Second { get; set; }

        [Parameter(ParameterSetName = "Set1")]
        public SwitchParameter ListAvailable { get; set; }
    }

An assembly containing this cmdlet would fail to be imported. It would also fail to execute, when no parameters were passed, since the parameter binding would be considered ambiguous as the default parameter set was ignored due to it being the AllParameterSets.
```