Special characters in field names should be escaped

This code:
```rust
use serde::Serialize;

#[derive(Default, Serialize)]
struct Foo {
    #[serde(rename = "bar\"")]
    bar: String,
}

fn main() {
    println!("{}", serde_json::to_string_pretty(&Foo::default()).unwrap());
}
```
... returns:
```json
{
  "bar\"": ""
}
```
... but Doku will document it in a wrong way:
```javascript
{
  "bar"": "string"
}
```