title: NMock : redefine method expectation
body: <p>I'm new to NMock and mocking in general.
  Is it possible to redefine an expectation ?
  In fact, I whant to mock an interface with many methods.
  So I decided to factorize common method expectations not to have to write them 1000 times.
  My issue is the following :
  I have a method to stub that loads a file.
  In most cases, the mock will do nothing for this method.
  So I factorized the expectation in [SetUp]</p>
  
  <pre><code>Stub.On(myMock).Method("Load").Will(Return.Value(True));
  </code></pre>
  
  <p>But, In a test case, I want to test that the object using the mock responds well to an exception, so I put in my test method :</p>
  
  <pre><code>Stub.On(myMock).Method("Load").Will(Throw.Exception(new FileNotFoundException()));
  </code></pre>
  
  <p>When I debug the test, I see the Load method is returning True.
  I can anderstand this, but is it possible to reset exceptation for a method or redefine it ?</p>