Add functions for ruby dummy encoding

- init_encoding_conversion()
- rbstr_dummyenc_new()
- rbstr_dummyenc_new_cstr()

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2558 f4f36d40-9719-0410-9868-dde021a23eda
Fix test_dataUsingEncoding_jis fails on ruby-2.x

git-svn-id: https://svn.code.sf.net/p/rubycocoa/svn/trunk/src@2559 f4f36d40-9719-0410-9868-dde021a23eda