title: URL init(string:relativeTo:) works wrong
body: <p>I'm trying to build a compound url in 3 steps, but the result is wrong!</p>
  
  <pre><code>// 1)
  let baseURL = URL(string: "http://clapps.clappsa.com")!
  print(baseURL.absoluteString)
  // http://clapps.clappsa.com
  
  // 2)
  let extendedURL = baseURL.appendingPathComponent("public", isDirectory: true)
  print(extendedURL.absoluteString)
  // http://clapps.clappsa.com/public/
  
  // 3)
  let finalURL = URL(string: "/img/3-mentee.jpg", relativeTo: extendedURL)!
  print(finalURL.absoluteString)
  // http://clapps.clappsa.com/img/3-mentee.jpg
  </code></pre>
  
  <p>Expected result:</p>
  
  <pre><code>http://clapps.clappsa.com/public/img/3-mentee.jpg
  </code></pre>
  
  <p>Even if I try to use the <code>absoluteURL</code> of the <code>extendedURL</code> like this:</p>
  
  <pre><code>let finalURL = URL(string: "/img/3-mentee.jpg", relativeTo: extendedURL.absoluteURL)!
  print(finalURL.absoluteString)
  // http://clapps.clappsa.com/img/3-mentee.jpg
  </code></pre>
  
  <p>I will get the same result.</p>
  
  <p>Strange, but such approach works ok with some other different URLs. </p>