test_rational.rb: fix default external

* test/ruby/test_rational.rb (test_conv): fix default external
  encoding, which affects inspect results.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51830 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_rational.rb

@@ -110,7 +110,11 @@ class Rational_Test < Test::Unit::TestCase
     assert_equal(Rational(1),Rational('3/3','3/3'))
     assert_raise(TypeError){Rational(nil)}
     assert_raise(ArgumentError){Rational('')}
-    assert_raise_with_message(ArgumentError, /\u{221a 2668}/) {Rational("\u{221a 2668}")}
+    assert_raise_with_message(ArgumentError, /\u{221a 2668}/) {
+      EnvUtil.with_default_external(Encoding::UTF_8) {
+        Rational("\u{221a 2668}")
+      }
+    }
     assert_raise(TypeError){Rational(Object.new)}
     assert_raise(ArgumentError){Rational()}
     assert_raise(ArgumentError){Rational(1,2,3)}