DateTaime.IsZero() should support pointer receiver

[pavel-boldyrev-form3]

I think this is a regression after #105, particularly this line:

func (t DateTime) IsZero() bool {
	return time.Time(t).IsZero()
}

I think IsZero() func should accept a pointer receiver, as it may be called on nil values.

For example, I have a generated code from a swagger model that does the following check on an optional date-time field created_on in the request:

func (m *Entity) validateCreatedOn(formats strfmt.Registry) error {
	if swag.IsZero(m.CreatedOn) { // not required
		return nil
	}

	if err := validate.FormatOf("created_on", "body", "date-time", m.CreatedOn.String(), formats); err != nil {
		return err
	}

	return nil
}

which now panics on the very first if statement.

[piepmatz]

Alternative: go-openapi/swag#67