Preserve query params in page iterator

The page iterator is losing my original query params on subsequent queries. In one particular case, I was passing `since` and `until` while listing commits in a repo, and the params were subsequently lost.

First query:
```
EXECUTED: get - /repos/heroku/darwin/commits with {"since"=>"2013-02-01T00:00:00Z", "until"=>"2013-02-09T01:34:16Z"} and {}
```

Next query:
```
EXECUTED: get - https://api.github.com/repos/heroku/darwin/commits with {"sha"=>"21d474f43f8edf67abb757380f748f48bc44e77c", "per_page"=>30} and {}
```

There are two paths in `PageIterator#next` - the first path is not preserving the params. This change preserves the params and does the `sha` param setting (without removing `last_sha`.

Seems to be working for me now.