Remove extra Content-Length HTTP header

The send_request_raw already sets the header and if it's set also in the
module, Metasploit sends the header twice.

modules/exploits/unix/webapp/oscommerce_filemanager.rb

@@ -82,7 +82,6 @@ class Metasploit3 < Msf::Exploit::Remote
       'headers' =>
       {
         'Content-Type'	 => 'application/x-www-form-urlencoded',
-        'Content-Length' => data.length,
       }
     }, 3)