title: Using docutils.core.publish_string instead of publish_cmdline, `settings_overrides` argument carrying HTML-Specific Options takes no effect!
body: <p>Below is my code</p>
  
  <pre><code>from docutils.core import publish_string
  from docutils.writers.html4css1 import Writer as HisWriter
  
  args = {
      'stylesheet' : '/home/wonder/lab/css/note.css',
      'stylesheet-path' : None,
  }
  
  src = 'ccav'
  print publish_string(src, writer=HisWriter(), settings_overrides=args)
  </code></pre>
  
  <p>I got the following error:</p>
  
  <pre><code>AssertionError: stylesheet and stylesheet_path are mutually exclusive.
  </code></pre>
  
  <p>So, I change <code>args</code> to:</p>
  
  <pre><code>args = {
      'stylesheet-path' : '/home/wonder/lab/css/note.css',
      'stylesheet' : None,
  }
  </code></pre>
  
  <p>Now, There is no errors. But, The stylesheet inserted into the HTML output is not the content of <code>/home/wonder/lab/css/note.css</code>. It is still <code>/usr/local/lib/python2.7/dist-packages/docutils/writers/html4css1/html4css1.css</code>.</p>
  
  <p>That is to say, unlike specify options in command line when using publish_cmdline, the <code>settings_overrides</code> argument carrying HTML-Specific Options takes no effect when using publish_string.</p>