default chalice http test client with application/json header

```python
dataset_response = chalice_client.http.post(
'/datasets',
headers={"content-type": "application/json"},
body=json.dumps(dataset))
```

It would be nice to just default having that header.

Start by tracing the chalice_client.http.post entry point used in the issue example and inspect how request headers are assembled. Check the existing HTTP client tests, if present, for POST requests and JSON bodies; done means callers can omit the content-type header while JSON requests still receive it without breaking explicitly supplied headers.

Written by the indexing model from the issue text.