fixed Pathname + method: if subclassing Pathname, when using + method…

… on instance of that subclass, method returns instance of Pathname, not instance of subclass

ext/pathname/lib/pathname.rb

@@ -305,8 +305,8 @@ def ascend
   # This method doesn't access the file system; it is pure string manipulation.
   #
   def +(other)
-    other = Pathname.new(other) unless Pathname === other
-    Pathname.new(plus(@path, other.to_s))
+    other = self.class.new(other) unless Pathname === other
+    self.class.new(plus(@path, other.to_s))
   end
 
   def plus(path1, path2) # -> path

test/pathname/test_pathname.rb

@@ -30,6 +30,9 @@ def self.defassert(name, result, *args)
   DOSISH_DRIVE_LETTER = File.dirname("A:") == "A:."
   DOSISH_UNC = File.dirname("//") == "//"
 
+  class SuperPathname < Pathname
+  end
+
   def cleanpath_aggressive(path)
     Pathname.new(path).cleanpath.to_s
   end
@@ -204,6 +207,10 @@ def plus(path1, path2) # -> path
 
   defassert(:plus, 'a//b/d//e', 'a//b/c', '../d//e')
 
+  define_assertion(:plus) {
+    assert_instance_of(SuperPathname, SuperPathname.new('a') + 'b')
+  }
+
   def test_parent
     assert_equal(Pathname("."), Pathname("a").parent)
   end