Issue Description:

The behavior of URL requests for a `POST` method with no post body and no `Content-type` header is inconsistent between MacOS and Linux.

In Linux, a `Content-type` header is added to the request:
```
Content-Type:application/x-www-form-urlencoded
```

In MacOS, no `Content-type` header is added.

Note that this behavior is for the specific case of no post body – i.e. the `content-length` is 0. When `content-length` is > 0, both platforms add the Content-type header when none is specified.

The following simple test case illustrates the problem (thanks to Nethra Ravindran):
```
import Foundation
import Dispatch

var req = URLRequest(url: URL(string: "http://httpbin.org/post")!)
req.httpMethod = "POST"

let urlSession = URLSession(configuration: URLSessionConfiguration.default)
let dataTask = urlSession.dataTask(with: req) { data, response, error in
print(String(data: data))
exit(0)
}
dataTask.resume()
dispatchMain()
```