title: stdlib Pathname#relative_path_from does not support mixed directory separators on windows
body: <p>The support for mixing backslashes and forward slashes in the pathname module in the standard library seems to be broken on windows</p>
  
  <pre><code>require 'pathname'
  base = Pathname.new("c:\\")
  filepath = Pathname.new("c:/foo/bar/file.ext")
  base.relative_path_from(filepath)
  </code></pre>
  
  <p>raises <code>ArgumentError: different prefix: "c:\\" and "c:/foo/bar/file.ext"</code></p>
  
  <p>Changing filepath to contain backslashes fixes the problem</p>
  
  <pre><code>filepath = Pathname.new("c:\\foo\\bar\\file.ext")
  base.relative_path_from(filepath)
  </code></pre>
  
  <p><code>=&gt; #&lt;Pathname:../../..&gt;</code></p>