Updated Columns unit tests to set and unset data_type
Updated Columns unit tests to set and unset data_type

vascotenner/holoviews,mjabri/holoviews,ioam/holoviews,mjabri/holoviews,ioam/holoviews,ioam/holoviews,vascotenner/holoviews,basnijholt/holoviews,basnijholt/holoviews,mjabri/holoviews,basnijholt/holoviews,vascotenner/holoviews