io.c: conversion from bom encoding

* io.c (rb_io_ext_int_to_encs, parse_mode_enc): bom-prefixed name is
  not a real encoding name, just a fallback.  so the proper conversion
  should take place even if if the internal encoding is equal to the
  bom-prefixed name, unless actual encoding is equal to the internal
  encoding.  [ruby-core:54563] [Bug #8323]
* io.c (io_set_encoding_by_bom): reset extenal encoding if no BOM
  found.  [ruby-core:54569]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40462 b2dd03c8-39d4-4d8f-98ff-823fe69b080e