Ruby

• transcode.c (rb_econv_conv): new function. it don't consume input
  too much, even for multilevel conversion.
  (transcode_loop): use rb_econv_conv.
  (econv_primitive_convert): ditto.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@18610 b2dd03c8-39d4-4d8f-98ff-823fe69b080e