[RFR] Fix DashboardMenuItem className prop

[Kmaschta]

Problem

When (re)using the component <DashboardMenuItem>, if pass it a className prop it is supressed by the component and not passed to its children MenuItem.

Solution

1. Pass the className prop to its children
2. or, let this one in the default props passed in the children

I choose the second one, because this prop isn't used anyway.

[fzaninotto]

Thanks!