[Bug #18998] Honor `#to_str` next to `#to_int` in `Kernel#Integer`

object.c

@@ -3138,6 +3138,9 @@ rb_convert_to_integer(VALUE val, int base, int raise_exception)
     tmp = rb_protect(rb_check_to_int, val, NULL);
     if (RB_INTEGER_TYPE_P(tmp)) return tmp;
     rb_set_errinfo(Qnil);
+    if (!NIL_P(tmp = rb_check_string_type(val))) {
+        return rb_str_convert_to_inum(tmp, base, TRUE, raise_exception);
+    }
 
     if (!raise_exception) {
         VALUE result = rb_protect(rb_check_to_i, val, NULL);

test/ruby/test_integer.rb

@@ -247,6 +247,13 @@ class TestInteger < Test::Unit::TestCase
     end;
   end
 
+  def test_Integer_when_to_str
+    def (obj = Object.new).to_str
+      "0x10"
+    end
+    assert_equal(16, Integer(obj))
+  end
+
   def test_int_p
     assert_not_predicate(1.0, :integer?)
     assert_predicate(1, :integer?)