Support passing a #to_str object to Pathname.new for compatibility

* See https://github.com/ruby/pathname/pull/57#issuecomment-3485646510

pathname_builtin.rb

@@ -217,7 +217,8 @@ class Pathname
   def initialize(path)
     unless String === path
       path = path.to_path if path.respond_to? :to_path
-      raise TypeError unless String === path
+      path = path.to_str if path.respond_to? :to_str
+      raise TypeError, "Pathname.new requires a String, #to_path or #to_str" unless String === path
     end
 
     if path.include?("\0")

test/pathname/test_pathname.rb

@@ -484,6 +484,10 @@ class TestPathname < Test::Unit::TestCase
     assert_equal('a', p1.to_s)
     p2 = Pathname.new(p1)
     assert_equal(p1, p2)
+
+    obj = Object.new
+    def obj.to_str; "a/b"; end
+    assert_equal("a/b", Pathname.new(obj).to_s)
   end
 
   def test_initialize_nul