Ensure decoding as datetime64[ns]

Pandas seems to have trouble constructing multi-indices when it's given
datetime64 arrays which don't have ns precision. The current version of
decode_cf_datetime will give datetime arrays with the default precision, which
is us. Hence, when coupled with the dtype restoring wrapper from PR #54, the
`to_series()` and `to_dataframe()` methods were broken when using decoded
datetimes.
Fix encoding for datetime64[ns]