Prefer Body attribute in request if provided

Take a simple document like this one:

```
FORMAT: 1A
HOST: https://cold.pizza

# Some API

# Root [POST /foo]

+ Request (application/xml)

+ Attributes (foo)

+ Body

something

+ Response 200 (application/xml)

+ Attributes (result)

+ Body

OK

## Data Structures

### `result`

- result (status, required)

### `status`

- status: OK (string, required)

### `foo`

- foo (required, bar)

### `bar`

- bar: woof (required, string)
```

When rendered using the default template (is this different with any others?) both the request and the response elements render the body as JSON instead of XML, even though the `content-type` header is correctly detected.


Note also that the example string for the rendered body (`"woof"`) is taken from the `Data Structures` section instead of the provided Body (in which case it would be `"something"`).

I propose two changes to fix this:

* Generate a request body from the attributes _only_ when no Body has been provided already (we can assume that if the user provided one it's because they wanted it).

* When generating a body, take into consideration what the content-type is for that request. There are probably too many to get it right, but we could definitely do better than forcing everything to use JSON no matter what.