title: FTP.delete(filename) from ftplib error
body: <p>When I try to use <code>ftp.delete()</code> from ftplib, it raises <code>error_perm</code>, resp:</p>
  
  <pre><code>&gt;&gt;&gt; from ftplib import FTP
  &gt;&gt;&gt; ftp = FTP("192.168.0.22")
  &gt;&gt;&gt; ftp.login("user", "password")
  '230 Login successful.'
  &gt;&gt;&gt; ftp.cwd("/Public/test/hello/will_i_be_deleted/")
  '250 Directory successfully changed.'
  &gt;&gt;&gt; ftp.delete("/Public/test/hello/will_i_be_deleted/")
  ...
  Traceback (most recent call last):
  File "&lt;stdin&gt;", line 1, in &lt;module&gt;
  File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/ftplib.py", line 520, in delete
  resp = self.sendcmd('DELE ' + filename)
  File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/ftplib.py", line 243, in sendcmd
  return self.getresp()
  File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/ftplib.py", line 218, in getresp
  raise error_perm, resp
  ftplib.error_perm: 550 Delete operation failed.
  </code></pre>
  
  <p>The directory exists, and "user" has sufficient permissions to delete the folder.</p>
  
  <p>The site is actually a NAS (WD MyBookWorld) that supports ftp.</p>
  
  <p>Changing to parent directory and using command <code>ftp.delete("will_i_be_deleted")</code> does not work either.</p>
  
  <p>"will_i_be_deleted" is an empty directory.</p>
  
  <p>ftp settings for WD MyBookWorld:</p>
  
  <pre><code>Service - Enable; Enable Anonymous - No; Port (Default 21) - Default
  </code></pre>