How to stub out a library file

Hi all,

I encounter a problem when i'm stubing out a library file.
The rails version is 2.3.4.

[How to reproduce]
1. Create a file payment_gateway.rb in subdirectory lib/.
[code]
class PaymentGateway
  def self.collect
    p 'lib/payment_gateway'
    true
  end
end

2. Create a file payment_gateway.rb in subdirectory test/mocks/test.
[code]
require 'lib/util/payment_gateway'

class PaymentGateway
  def self.collect
    p 'mock/payment_gateway'
    false
  end
end

3. Create a method in any unit test file to test this method.

The actual result is invoking the method in the mock file, and the
output is
'mock/payment_gateway'
false

But if i add the following code in the application_controller.rb
[code]
require 'util/payment_gateway'

The actual result is invoking the method in the lib file, and the output
is
'lib/payment_gateway'
true

I don't know why is that, could anyone give me some help?

Thanks very much.