Fix _set_default_controller() not to look in sub-folders

_set_default_controller() sets the class and method directly.
That's ok, because we haven't actually defined what it
would mean to have a default route pointing to a sub-folder.
(Does the URL /directory/ cause us to look in a sub-sub-folder?
Could we get into an infinite loop?)

But then it goes ahead and call _set_request(), which may
try to look in sub-folders.  It would then re-set the class
 - and the method.  Unless the default route doesn't specify
a method, in which case it would set a new class but preserve
the old method, producing an inconsistent request.

Split the part of _set_request() we _do_ want into
_set_request_novalidate(), and call that instead.

system/core/Router.php

@@ -179,31 +179,25 @@ protected function _set_default_controller()
 		{
 			show_error('Unable to determine what should be displayed. A default route has not been specified in the routing file.');
 		}
-		// Is the method being specified?
-		if (strpos($this->default_controller, '/') !== FALSE)
-		{
-			$x = explode('/', $this->default_controller);
-			$this->set_class($x[0]);
-			$this->set_method($x[1]);
-			$this->_set_request($x);
-		}
-		else
+		$segments = explode('/', $this->default_controller);
+
+		if ( ! file_exists(APPPATH.'controllers/'.$segments[0].'.php'))
 		{
-			$this->set_class($this->default_controller);
-			$this->set_method('index');
-			$this->_set_request(array($this->default_controller, 'index'));
+			show_404($segments[0]);
 		}
+		$this->_set_request_novalidate($segments);
 
 		log_message('debug', 'No URI present. Default controller set.');
 	}
-
+	
 	// --------------------------------------------------------------------
 
 	/**
 	 * Set the Route
 	 *
 	 * This function takes an array of URI segments as
-	 * input, and sets the current class/method
+	 * input, sets the current class/method, and makes
+	 * sure we know how to find the class.
 	 *
 	 * @param	array
 	 * @return	void
@@ -217,6 +211,22 @@ protected function _set_request($segments = array())
 			return $this->_set_default_controller();
 		}
 
+		$this->_set_request_novalidate($segments);
+	}
+
+	// --------------------------------------------------------------------
+
+	/**
+	 * Set the Route
+	 *
+	 * This function takes an array of URI segments as
+	 * input, and sets the current class/method
+	 *
+	 * @param	array
+	 * @return	void
+	 */
+	protected function _set_request_novalidate($segments)
+	{
 		$this->set_class($segments[0]);
 
 		if ( ! isset($segments[1]))