Use ToArrayInterface type check when accessing response headers

I'm experiencing BadResponseExceptions when using [this client](https://github.com/rdohms/meetup-api-client) because a HeaderCollection is being passed into the `Http\Message\Response` class constructor - this may have been introduced by the recent HeaderCollection changes? I've altered the constructor to check for `Guzzle\Common\ToArrayInterface` instead, which should cater for all cases.