How about append default header for axios and typescript style.

Hi there, very thanks to make js interface from type based safety api!

I used 'type to axios style api', and thought that it is more useful for development(flexibility) to inject default header "Content-value 'application/json'" and body as {}(this maybe axios behavior likely bug. When none body is passed, header is not inject if header value is passed.).

How about inject axios default header Content-Type 'application/json' and body as {} or merged with body like {...body}?
Example output like below.
```
export interface XRequest {
xId: XID;
message: string;
}

// this setting supports cors(of course when server support)
export const postX = (body: XRequest) => {
return axios.post(
endpoint + "/conv",
{ ...body },
{
headers: {
"Content-Type": "application/json"
}
}
);
};
```

I think it is easy to fix and provide more seamless work for api with client lib also under development environment.