Stored scripts API accepting malformed requests

The stored scripts API today accepts malformed requests instead of throwing an exception.  For instance:

```
POST _scripts/foo
{
  "source": "return"
}

GET _scripts/foo
```

returns

```
{
  "_id": "foo",
  "found": true,
  "script": {
    "lang": "mustache",
    "source": """{"source":"return"}"""
  }
}
```