Catch RedirectNotAllowed correctly

The error `RedirectNotAllowed` is wrapped by `url.Error`, causing it to not
currently be caught/ignored correctly for responses that issue redirects:
```
2014/01/06 12:17:17 Request error: Get https://example.com/foo: Redirects not allowed
```

It seems that two type assertions/conditions are required to unravel the
exact error. I can't see any way to write a test for this functionality --
the call to `defer()` or `log.Println()`.
Merge pull request #70 from buger/catch_redirect_errors

Catch RedirectNotAllowed correctly