fix decay function
fix decay function

decay function was not nesting the origin inside a field dict

the json send was:

```json
{"gauss": {
	"field": "my_field",
	"origin": {
		"lat":42,
		"lon":24
	},
	"scale":"3km"
}}
```

now it's

```json
{"gauss": {
	"my_field": {
		"origin": {
			"lat":42,
			"lon":24
		},
		"scale":"3km"
	}
}}
```

benashford/rs-es