Change NSURL.description to match Darwin

[jaysk1]

Consider this program

import Foundation

let nsurl = NSURL(string: "http://google.com")!
let url = URL(string: "http://amazon.in")!

print(url)
print(nsurl)

Output on Darwin

http://amazon.in
http://google.com

Output on Linux

<CFURL 0x8eb3ed0 [0x7f7f63d54c38]>{string = http://amazon.in, encoding = 134217984, base = (null)}
<CFURL 0x8e41620 [0x7f7f63d54c38]>{string = http://google.com, encoding = 134217984, base = (null)}

This PR makes the output on Linux same as that on Darwin.

[ianpartridge]

Thanks! Can we add a test too?

[jaysk1]

@ianpartridge : Added Test case to test URL.description under TestURL.swift.

[alblue]

@swift-ci please test and merge

[pushkarnk]

@swift-ci please test and merge