uri: load JSON for content-type: application/json

If the resource consumed by URI returns JSON, load JSON from the returned content string.

Sample playbook:

```yaml
  - action: uri
            url="http://localhost/jp.cgi"
            dest=/tmp/ph
            return_content=true
    register: body
  - action: template src=t1.j2 dest=/tmp/t1
```

Example template:

```
name = {{ body['json']['name'] }}
```

Example CGI

```sh
#!/bin/sh

echo "Content-type: application/json"
echo ""
echo '{ "name": "Jane Jolie", "number" : 49 }'
```