Fix hc:PasswordBox ContextMenu binding issue.

[GF-Huang]

Test Code

<hc:PasswordBox hc:InfoElement.ShowClearButton="True" Password="123456" IsSafeEnabled="False">
    <hc:PasswordBox.ContextMenu>
        <ContextMenu>
            <MenuItem Command="ApplicationCommands.Copy" />
            <MenuItem Command="ApplicationCommands.Cut" />
            <MenuItem Command="ApplicationCommands.Paste" />
        </ContextMenu>
    </hc:PasswordBox.ContextMenu>
</hc:PasswordBox>