Issue Description:

While debugging an issue when using Alamofire 5 on Linux I discovered the root cause to be a difference in behavior between Darwin and Linux Foundation when retrieving a url from an existing URLRequest (or NSURLRequest) for relative URL's. Consider the following code:

import Foundation

let baseUrl = URL(string: "http://httpbin.org")
let url = URL(string: "/get", relativeTo: baseUrl)!

print("url = (url)")
print("base = (url.baseURL!)")
print("relativeString = (url.relativeString)")
var req = URLRequest(url: url)
print("req.url = (req.url!)")
req.url = url
print("req.url = (req.url!)")

var nsreq = NSURLRequest(url: url)
print("nsreq.url = (nsreq.url!)")

Darwin output:
url = /get – http://httpbin.org
base = http://httpbin.org
relativeString = /get
req.url = http://httpbin.org/get
// reassignment happens here
req.url = /get – http://httpbin.org
nsreq.url = http://httpbin.org/get

Ubuntu output:
url = /get – http://httpbin.org
base = http://httpbin.org
relativeString = /get
req.url = /get – http://httpbin.org
// reassignment happens here
req.url = /get – http://httpbin.org
nsreq.url = /get – http://httpbin.org

Notice the difference in output. It looks like the .url property on Darwin is assigned a resolved absolute URL when NSURLRequest and URLRequest objects are constructed, however, a reassignment to .url property preserves the relative URL.