Fix variable responses for json body

Canned was unable to manage variable responses when request body contains json data.

**Request body**
```
{"login":"lina.duval"}
```

**index.post.json**

````
//! body: {"login":"Adrien62"}
{
    "id": 42,
    "label" : "MENARD Adrien"
}
//! body: {"login":"lina.duval"}
{
    "id": 43,
    "label" : "DUVAL Lina"
}
````

**Result is KO**
 ```
{
    "id": 42,
    "label" : "MENARD Adrien"
}
```
I fixed the problem by parsing json data when request content type is *application/json*.