transcode.c: use string

* transcode.c (econv_opts): get rid of inadvertent ID creation,
  and preserve argument encoding in error messages.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47041 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_econv.rb

@@ -908,6 +908,9 @@ class TestEncodingConverter < Test::Unit::TestCase
     ec1 = Encoding::Converter.new("", "", universal_newline: true)
     ec2 = Encoding::Converter.new("", "", newline: :universal)
     assert_equal(ec1, ec2)
+    assert_raise_with_message(ArgumentError, /\u{3042}/) {
+      Encoding::Converter.new("", "", newline: "\u{3042}".to_sym)
+    }
   end
 
   def test_default_external

transcode.c

@@ -2487,8 +2487,8 @@ econv_opts(VALUE opt, int ecflags)
 	    /* ecflags |= ECONV_LF_NEWLINE_DECORATOR; */
 	}
 	else if (SYMBOL_P(v)) {
-	    rb_raise(rb_eArgError, "unexpected value for newline option: %s",
+	    rb_raise(rb_eArgError, "unexpected value for newline option: %"PRIsVALUE,
-		     rb_id2name(SYM2ID(v)));
+		     rb_sym2str(v));
 	}
 	else {
 	    rb_raise(rb_eArgError, "unexpected value for newline option");