set: fix Set#to_set block behavior and add regression test

Author: rwstauner

set.c

@@ -655,7 +655,7 @@ set_i_to_set(VALUE set)
         return set;
     }
 
-    return rb_funcall_passing_block(rb_cSet, id_new, 0, NULL);
+    return rb_funcall_passing_block(rb_cSet, id_new, 1, &set);
 }
 
 /*

test/ruby/test_set.rb

@@ -978,7 +978,8 @@ def test_to_set
     assert_equal([-10,-8,-6,-4,-2], set.sort)
 
     assert_same set, set.to_set
-    assert_not_same set, set.to_set { |o| o }
+    transformed = set.to_set { |o| o + 1 }
+    assert_equal([-9,-7,-5,-3,-1], transformed.sort)
   end
 
   class MyEnum