API: convert a masked Table to an unmasked one

I can convert an unmasked table by calling `tab_masked = Table(tab_unmasked , masked = True)`. However, the converse is not true. It seems odd to me, that a table get upgraded to masked, even if I explicetly specify `masked = False`.
```
>>> from astropy.table import Table
>>> tab0 = Table([[3,4]], masked = False)
>>> tab1 = Table(tab0, masked = True)
>>> tab2 = Table(tab1, masked = False)
INFO: Upgrading Table to masked Table [astropy.table.table]
```
I suggest that in this case a warning be issued, but the `tab2` should be unmasked (either with filled values or just with the values "behind the mask").
I know I can do  `tab2 = Table(tab._data.data)` but that strikes me as a very "hacky" way to do it.

(Related: There is a top level `tab1.mask`, but no `tab1.data`. Is that a bug or a feature?
An intermediate solution would be to have a `tab1.data` similar to `tab1.mask`, which would allow `tab2 = Table(tab1.data)`).