Handle CSV files with "duplicate field" by reading the values in a vector

I've just encountered the same problem as was described in #353. I was wondering about a different solution:

Given a header with duplicate fields:

```
name,other,name'
```

I wonder if the values could be deserialized into a vector. Assuming this is the definition of the struct:

```rust
#[derive(Deserialize, Debug)]
struct Message {
name: Vec,
other: String,
}
```