use String#encode at scm_iconv on Ruby 1.9 (#12228)

use String#encode at scm_iconv on Ruby 1.9 (#12228)

Unit git adapter test fails on JRuby 1.7.

<pre>
Failure:
  <"test-Ü-2.txt">("UTF-8") expected but was
  <"test-\xC3\x9C-2.txt">("ASCII-8BIT").

  diff:
  - test-Ü-2.txt
  ?      ^
  + test-Ü-2.txt
  ?      ^^
test_entries_latin_1_files(GitAdapterTest)
test/unit/lib/redmine/scm/adapters/git_adapter_test.rb:468:in `test_entries_latin_1_files'
     465:         assert entries1
     466:         assert_equal 3, entries1.size
     467:         f1 = entries1[1]
  => 468:         assert_equal "test-#{@char_1}-2.txt", f1.name
     469:         assert_equal "latin-1-dir/test-#{@char_1}-2.txt", f1.path
     470:         assert_equal 'file', f1.kind
     471:       end
</pre>

git-svn-id: svn+ssh://rubyforge.org/var/svn/redmine/trunk@10740 e93f8b46-1217-0410-a6f0-8f06a7374b81