Make StatAutodensity and StatAutobin work if the data has NA values

[ilia-kats]

This closes #174. I'm not sure if this is the best approach, but as far as I understand, ggplot always removes NA values, the only difference is whether there is a user-visible warning or not (controlled by the na.rm parameter). Since the warning is produced in compute_layer() in the Stat base class, it should not be affected by this change.