Fixed Asserts.Equal, expected and actual were around the wrong way

I noticed while writing some failing tests using the ShouldContainAttribute extension method that the expected and actual values have been incorrectly specified in the Nancy.Testing source code.

The attached Test Failure is from the following assertion:

response.Body["form#frm1 input[name=Id]"].ShouldExistOnce().And.ShouldContainAttribute("value", "cea085d7-4106-4a69-844b-2f3fc3404dbb");

![image](https://cloud.githubusercontent.com/assets/2233210/3399286/98c012a4-fd3a-11e3-9630-4177000e3f82.png)