title: TwitterAPI account/update_profile_background_image not works
body: <p>I'm using Abraham's PHP library to connect with the twitter api.
  Im trying to update the background profile, after authentication doing the following:</p>
  
  <pre><code>$oauth = new TwitterOAuth($this-&gt;tw_key, $this-&gt;tw_secret, $this-&gt;tw_current_accessToken, $this-&gt;tw_current_accessSecret);
  $oauth-&gt;host = $this-&gt;tw_host;
  $result = $oauth-&gt;post('account/update_profile_background_image', array('image' =&gt; base64_encode(file_get_contents($image_full_path))));
  </code></pre>
  
  <p>Path to image is correct.<br/>
  I can do a <code>verify_credentials</code> perfectly, so the auth is not a problem.<br/>
  It returns json coded array, like all is fine, but the background image param is not changing.<br/>
  If i'm doing this using <code>array('image' =&gt; '@'.$image_full_path)</code> it gives 131 internal error, maybe because Abraham's library does not support multipart image uploads.</p>
  
  <p>All other methods, like <code>account/update_profile_image</code> are working fine.<br/>
  Any ideas? Thanks.</p>