Deserializing a String field inside a flattende struct fails if the field contains a valid integer

#### What version of the `csv` crate are you using?

1.3.0

#### Briefly describe the question, bug or feature request.

When deserializing to a `String` field using serde, `csv` can handle a field containing `123`.
However, when doing the same to a flattened field, `csv` can no longer deserialize `123` to the `String` field and I get the error:
```
Err(
Error(
Deserialize {
pos: Some(
Position {
byte: 24,
line: 4,
record: 3,
},
),
err: DeserializeError {
field: None,
kind: Message(
"invalid type: integer `123`, expected a string",
),
},
},
),
)
```

#### Include a complete program demonstrating a problem.

```rust
use serde::Deserialize;

#[allow(dead_code)]
#[derive(Debug, Deserialize)]
struct Inner {
inner_str: String,
}

#[allow(dead_code)]
#[derive(Debug, Deserialize)]
struct Row {
str: String,

#[serde(flatten)]
inner: Inner,
}

fn main() {
let source = r#"
str,inner_str
A,A
123,A
A,123
"#
.trim();

let mut reader = csv::Reader::from_reader(source.as_bytes());
for line in reader.deserialize::() {
dbg!(&line);
}
}
```

#### What is the observed behavior of the code above?

Output:
```
[src/main.rs:29] &line = Ok(
Row {
str: "A",
inner: Inner {
inner_str: "A",
},
},
)
[src/main.rs:29] &line = Ok(
Row {
str: "123",
inner: Inner {
inner_str: "A",
},
},
)
[src/main.rs:29] &line = Err(
Error(
Deserialize {
pos: Some(
Position {
byte: 24,
line: 4,
record: 3,
},
),
err: DeserializeError {
field: None,
kind: Message(
"invalid type: integer `123`, expected a string",
),
},
},
),
)
```

#### What is the expected or desired behavior of the code above?

Output:
```
[src/main.rs:29] &line = Ok(
Row {
str: "A",
inner: Inner {
inner_str: "A",
},
},
)
[src/main.rs:29] &line = Ok(
Row {
str: "123",
inner: Inner {
inner_str: "A",
},
},
)
[src/main.rs:29] &line = Ok(
Row {
str: "A",
inner: Inner {
inner_str: "123",
},
},
)
```