Processing an FDD into a new FDD with different custom serializers is broken in 2.0.0

[tstandley]

Custom serializers are ignored when copying rows that aren't completely modified:

with RFDD('input.fdd', columns=('a','b')) as input, \
     WFDD('output.fdd', columns('a','b'), column_to_serialize(pkl.dumps, custom_s)) as output:
    for k,v in input.items():
        v.a=v.a+1
        output[k]=v

results in the custom_s serializer being ignored because the bits are being copied directly rather than doing a custom_s(deserialize(bits)) operation.

The user should at least be warned.