[Rd] as.Date.factor

[Gabor Grothendieck]

as.Date.character has a format= argument but as.Date.factor does not
which means that one can only use as.Date.factor if the factor labels have
the default Date format.  

as.Date.factor just does a

   # as.Date.factor currently does this
   as.Date(as.character(x)) 

but for consistency's sake I think it should pass additional arguments such
as format along:

   # as.Date.factor should do this
   as.Date(as.character(x,), ...)