Bug - HTTP Client can call :shutdown on closed IO

When running Rex HTTP client calls across pivots, pivot sockets
can get closed by the remote server, resulting in a closed :conn
object within the client object. The clients :close method calls
self.conn.shutdown which raises an 'IOError closed stream' on what
is effectively a TCPSocket object in a closed state (under the Rex
abstraction).

Resolve by moving the self.conn.closed? check into the conditional
just above the :shutdown call, and remove if from the underlying
:close call as calling :close on an already closed TCPSocket
returns nil as opposed to throwing an exception like the :shutdown
method.

lib/rex/proto/http/client.rb

@@ -191,9 +191,9 @@ class Client
   # Closes the connection to the remote server.
   #
   def close
-    if (self.conn)
+    if (self.conn and !self.conn.closed?)
       self.conn.shutdown
-      self.conn.close unless self.conn.closed?
+      self.conn.close
     end
 
     self.conn = nil