Allow any object type to be used for a partial update during upsert

Hi,

I have made a minor change to the Document() upsert API so that you can supply any object as the partial update (eg, a mutation object) and not require the strong object to be supplied for a partial update.

This is in reference to the upsert/doc API at http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/docs-update.html

There is a use case where I have a document type (say ShoppingCart) and I want to perform an upsert using a less-descriptive object type (eg, ShoppingCartMutation).

Previously this use case was not possible because of the type restriction on the Document() method that restricted what partial update object could be supplied:

```csharp
// Classes
class ShoppingCart
{
    public string Name;
    public double Total;
    public DateTime Created;
}

class ShoppingCartMutation
{
    public string Name;
    public double Total;
}

// Instance
var newCart = new ShoppingCart();
newCart.Created = DateTime.UtcNow;
newCart.Name = "New Cart";
newCart.Total = 0;

var mutation = new ShoppingCartMutation();
mutation.Total = 5;

// Example
client.Update<ShoppingCart>(x => x
    .Type(typeof(ShoppingCart))
    .Document(mutation)
    .Upsert(newCart)
);
```

Previously this would not compile because both Document() and Upsert() overloads needed to be of the same type.

If this was the case then I would need to always suppy a ShoppingCart instance for both arguments, which would overwrite the Created date (as its not nullable in this example) during a partial update when that is not intended.