fix #83
fix #83

This enables to delete previously set content using a void string
bradberger/angular-material-calendar,jsmodules/angular-material-calendar,jsmodules/angular-material-calendar,bradberger/angular-material-calendar,bradberger/angular-material-calendar,BitolaCo/angular-material-calendar,BitolaCo/angular-material-calendar,BitolaCo/angular-material-calendar,jsmodules/angular-material-calendar