Encode any values for CSV

Hi,

Thank you for merging the PR about CSV encoding. I found a bug in an edge case after it was merged.

If a model has columns with serialized object, they don't respond to "encode!" but can include strings to encode.

e.g.
`{a: "a", b: "あいうえお"}`

`["a", "あいうえお"]`

So my solution here is encode any values after convert it to string since the values are converted sooner or later in [CSV lib](https://github.com/ruby/ruby/blob/trunk/lib/csv.rb#L2021).
Could you consider to merge this fix?