Pathname: Have #relative_path_from accept String argument.

[Fix GH-1975]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65706 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/pathname/lib/pathname.rb

@@ -503,6 +503,7 @@ class Pathname
   # ArgumentError is raised when it cannot find a relative path.
   #
   def relative_path_from(base_directory)
+    base_directory = Pathname.new(base_directory) unless Pathname === base_directory
     dest_directory = self.cleanpath.to_s
     base_directory = base_directory.cleanpath.to_s
     dest_prefix = dest_directory

test/pathname/test_pathname.rb

@@ -291,9 +291,10 @@ class TestPathname < Test::Unit::TestCase
   end
 
   def relative_path_from(dest_directory, base_directory)
-    Pathname.new(dest_directory).relative_path_from(Pathname.new(base_directory)).to_s
+    Pathname.new(dest_directory).relative_path_from(base_directory).to_s
   end
 
+  defassert(:relative_path_from, "../a", Pathname.new("a"), "b")
   defassert(:relative_path_from, "../a", "a", "b")
   defassert(:relative_path_from, "../a", "a", "b/")
   defassert(:relative_path_from, "../a", "a/", "b")