Allow update to support additional parameters

I want to return `_source` from an update operation. The attached patch doesn't work, however:

```ruby
index = Stretcher::Server.new.index('foo').type('bar')
index.put('foo', {bar: 'baz'})
index.update('foo', {doc: {bar: 12345}}, :fields => ["_source"])
#<Hashie::Mash _id="foo" _index="foo" _type="bar" _version=2 ok=true>
```
Despite passing in the `fields` option as described in the [update API documentation](http://www.elasticsearch.org/guide/reference/api/update/), the full document _source isn't included. From the command line, however, the full document is returned.

Any ideas?