Added non-ASCII but convertible encoding case

Author: nobu

test/stringio/test_stringio.rb

@@ -195,6 +195,9 @@ def test_write_encoding_conversion
     s = StringIO.new.set_encoding(conversion_encoding)
     s.write(convertible)
     assert_equal(conversion_encoding, s.string.encoding)
+    s = StringIO.new.set_encoding(Encoding::UTF_8)
+    s.write("foo".force_encoding("ISO-8859-1"), convertible)
+    assert_equal(Encoding::UTF_8, s.string.encoding)
     all_assertions do |a|
       [
         inconvertible,