Fix url_ok bug, status codes are integers

This happens:
```r
url_ok("http://httpbin.org/status/200")
# [1] FALSE
```

Because status codes are converted to integers here:
https://github.com/hadley/httr/blob/26d986de55c2142e91921a1fd94ad01311e6ebf0/R/perform.R#L66
But the test is against  a real number:
https://github.com/hadley/httr/blob/26d986de55c2142e91921a1fd94ad01311e6ebf0/R/response-status.r#L155

Also added some simple tests for this.