throw `{error,badarg}` when execute sql, because of timestamp column

[TangMonk]

If the the results contain timestamp without time zone column, will throw the {error,badarg}, If I set the column to null, then works.

I thinks the reason is lua not support timestamp type, and maybe results contain json or array will throw error too.

for friendly to newbie, I think this problem should be documentation on readme.

[dergraf]

Thanks for mentioning this. We should document this or at least log a more understandable error message e.g. can't convert unsupported datatype ...

[zhuchunyu]

Mongodb has the same error with a date field