Decoder works with missing fields

Currently if a struct field is missing in the database, the reader will throw an error, e.g., `DecodingError(Unknown struct field represented_country)`.

[Related issue](https://github.com/mozilla/rust/issues/12794) with the Rust JSON decoder. Ideally the behavior of this API would be similar to whatever the final behavior of that API is.
