Write headers with serde when the collection is empty

On `csv 1.1.1`

When using `serde` to serialize a CSV file, the header is not written in the file if `Writer::serialize()` has not been called. This might seems logical but here is my problem: I'm using a standard format based on CSV which force us to write a file with the headers, even if there is no data associated.

```rust
use csv::*;
use serde::*;

#[derive(Serialize, Deserialize)]
struct Object {
id: u32,
name: &'static str,
}

fn main() {
let object = Object {
id: 1,
name: "some name",
};
let objects: Vec = vec![object]; // XXXXX
let mut writer = WriterBuilder::default()
.has_headers(true)
.from_path("foo.csv")
.unwrap();
for o in objects {
writer.serialize(o).unwrap();
}
}
```
The above program will output the expected file with the header.

```csv
id,name
1,some name
```

But, if you replace the line annotated with `XXXXX` with
```rust
let objects: Vec = vec![];
```

The resulting file will end up completely empty but I'd like to be able to at least produce.
```csv
id,name
```

Obviously, if I never tell the `Writer` object to do anything, nothing will happen. But I was thinking maybe a `Writer::serialize_header()` could be provided? In the above piece of code, I could call `writer.serialize_header()` just above the `for` loop.

Note: I'm not a rockstar of Rust but I'd be fine to try to implement it if you could provide some guidance. But first, I need to be sure I didn't miss a piece of documentation that would do what I want! And also if you're ok about the possibility to add such a new function and the design of the API.