MaterialTextField Error in Binding TextChangeCommand : System.InvalidCastException Message=Object must implement IConvertible.

## 🐛 Bug Report
Hi!

When I try to Bind TextChangeCommand and ContentPage is openning the exception is thrown.

`System.InvalidCastException Message=Object must implement IConvertible.`

### Expected behavior
Receive Old and New text typed on control in ViewModel Command method

### Reproduction steps
XAML:

`
`

ViewModel.cs
`public ICommand AmountTextCahangeCommand
{
get
{
return new Command((a) =>
{
_amountTextCahangeCommand(a);
});
}
}
`
### Configuration

**Version:** 1.7.4

**Platform:**
- [ ] :iphone: iOS
- [X ] :robot: Android
- [ ] :checkered_flag: WPF
- [ ] :earth_americas: UWP
- [ ] :apple: MacOS
- [ ] :tv: tvOS
- [ ] :monkey: Xamarin.Forms

Thank you!