ref-counts are not properly updated when materializing an object column

[st-pasha]

>>> import datatable as dt
>>> class A: pass
... 
>>> DT = dt.Frame(A=range(10), B=[A() for i in range(10)])
>>> DT.key = 'A'
>>> DT.to_list()
Segmentation fault: 11

[st-pasha]

The issue appears to be in the materialize() step, which is performed after key-ing.