add a page_size method to Restforce::Collection

I have a Salesforce query that returns many results using the lazy-fetched enumerable Restforce::Collection.  Something like 

```ruby
results = restforce_client.query("Select Id from #{sf_class}")
results.each_slice(1_000) do |slice| 
  delay_send_spam_email(slice)
end
```

Using each_slice with the SF page size seems very clean, and efficient, since I can fetch a single page of results, and only get the next page when I'm ready to process it.
But sometimes SF returns pages of 1k results, sometimes 2k, and I don't want to have to figure that out.

So I'd like to do results.each_slice(results.page_size) do |slice| ...