title: can't convert "[" with encoding "gb2312" to "utf-8" in ruby1.9.3
body: <p>I'm learning ruby and try to get the filename from a ftp server. The string I got was encoded in <strong><em>gb2312(simplified Chinese)</em></strong>, It's success in most cases with these codes:</p>
  
  <pre><code>str = str.force_encoding("gb2312")
  str = str.encode("utf-8")
  </code></pre>
  
  <p>but it will make an error <code>"in encode': "\xFD" followed by "\x88" on GB2312 (Encoding::InvalidByteSequenceError)"</code> if the string contains the symbol <code>"["</code> or <code>"【"</code>.</p>