Nullable<DataTime> causes CommandWpf to thorw exception on Execute

[Keboo]

Reproduction Steps:

1. Create an instance of GalaSoft.MvvmLight.CommandWpf.RelayCommand<DateTime?>
2. Invoke Execute() passing a DateTime instance for the parameter.

Observed:

System.InvalidCastException
  HResult=0x80004002
  Message=Invalid cast from 'System.DateTime' to 'System.Nullable`1[[System.DateTime, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]'.
  Source=mscorlib
  StackTrace:
   at System.Convert.DefaultToType(IConvertible value, Type targetType, IFormatProvider provider)
   at System.DateTime.System.IConvertible.ToType(Type type, IFormatProvider provider)
   at System.Convert.ChangeType(Object value, Type conversionType, IFormatProvider provider)
   at GalaSoft.MvvmLight.CommandWpf.RelayCommand`1.Execute(Object parameter) in C:\Dev\mvvmlight\GalaSoft.MvvmLight\GalaSoft.MvvmLight (PCL)\Command\RelayCommandGeneric.cs:line 221
   at GalaSoft.MvvmLight.Test.Command.RelayCommandGenericTest.TestNullableDateTimeConversionExecute() in C:\Dev\mvvmlight\GalaSoft.MvvmLight\Tests\PCL\GalaSoft.MvvmLight.Test (PNET45)\Command\RelayCommandGenericTest.cs:line 347

Expected: The command should execute.

The problem appears to be specific to the RelayCommand<T> when compiled with !NETFX_CORE.
Specifically this section in the RelayCommand<T>.Execute().

#if !NETFX_CORE
            if (parameter != null
                && parameter.GetType() != typeof(T))
            {
                if (parameter is IConvertible)
                {
                    val = Convert.ChangeType(parameter, typeof (T), null);
                }
            }
#endif

This is problematic when the generic parameter is Nullable and the passed in parameter is a a boxed instance of DateTime.

Unit Tests:

[TestMethod]
public void TestNullableDateTimeConversionCanExecute()
{
    var today = DateTime.Today;
    var command = new CommandWpf.RelayCommand<DateTime?>(
        x => _methodWasExecuted = true, 
        x => x == today);

    Assert.IsTrue(command.CanExecute(today));
}

[TestMethod]
public void TestNullableDateTimeConversionExecute()
{
    var today = DateTime.Today;
    DateTime? executedParameter = null;
    var command = new CommandWpf.RelayCommand<DateTime?>(
        x =>
        {
            executedParameter = x;
            _methodWasExecuted = true;
        });

    command.Execute(today);

    Assert.IsTrue(_methodWasExecuted);
    Assert.AreEqual(today, executedParameter);
}

Proposed fix:

#if !NETFX_CORE
            if (parameter != null && !(parameter is T) && parameter is IConvertible) 
            { 
                val = Convert.ChangeType(parameter, typeof (T), null); 
            }
#endif