title: ocmock's invokeBlockWithArgs using a nil argument
body: <p>is there any way to invoke a block with <code>nil</code> as a given argument, given that the <code>invokeBlockWithArgs:</code> requires the args to be nil-terminated?</p>
  
  <p>example method definition in a mocked object:</p>
  
  <pre><code>- (void)methodWithCompletion:(void(^)(NSString*, NSError* )) completionBlock;
  </code></pre>
  
  <p>The given mockObject should call:</p>
  
  <pre><code>completionBlock(@"foo", nil);
  </code></pre>
  
  <p>however, with <code>invokeBlockWithArgs</code>:</p>
  
  <pre><code>OCMStub([mockObj methodWithCompletion:([OCMArg invokeBlockWithArgs:@"foo", nil, nil])]);
  </code></pre>
  
  <p>Method fails, with too few arguments; obviously with nil being the termination, it doesn't recognize the second parameter to the block should be <code>nil</code>.</p>