Fix for issue 1675 on jruby-1_7: String#casecmp on UTF-16LE encoded string

This commit fixes issue #1675 on jruby-1_7 branch. I wrote a rspec test using the available encodings in addition to UTF-16LE. See *GH-1675_casecmp_on_UTF16LE_encoded_string_spec.rb* for details.