Overload the #update_issue method to allow partial updates

Motivation: my application allows users to perform partial updates on issues (e.g. only update the assignee of an issue). Instead of always fetching the issue first to pass back the title and body, I'd rather just update the single attribute. It feels to me like this functionality is inline with the spirit of the `PATCH` verb.

This implementation maintains backward compatibility, but allows the user to optionally omit the `title` and `body` positional arguments in favor of a single `options` hash:

```ruby
client.update_issue("octokit/octokit.rb", 123, :assignee => "foo")
```

Thoughts?