Use the published initialiser for NSHTTPURLResponse instead of the undocumented one.

In iOS 5.0 and OS X 10.7, Apple published `-[NSHTTPURLResponse initWithURL:statusCode:HTTPVersion:headerFields:]`.

This pull request uses that method instead of the private initialiser previously used. I have confirmed that the tests in `ILCannedURLProtocolTests.m` still pass with no changes.