ResultPager::get() can return string

Because `ResultPager::get()` [here](https://github.com/KnpLabs/php-github-api/blob/master/lib/Github/ResultPager.php#L205) returns the return value of [`ResponseMediator::getContent()`](https://github.com/KnpLabs/php-github-api/blob/master/lib/Github/HttpClient/Message/ResponseMediator.php#L11-L26), it can occasionally return a string, which causes an unhelpful TypeError since it should only ever return an array. Other usages of `ResponseMediator::getContent()` allow for this, but the `ResultPager` one does not.

This has been occurring in practice to us so figured I'd raise it here - let me know any other information you need on this issue, it is my first one!

Start in lib/Github/ResultPager.php at ResultPager::get() and compare its use of ResponseMediator::getContent() with the behavior described in lib/Github/HttpClient/Message/ResponseMediator.php. Confirm the method consistently returns an array, add coverage for the string-content case, and verify the resulting TypeError no longer occurs.

索引モデルが issue の本文から書いたものです。