Allow the SimpleHttpResponseProvider to have its mocks reset

Me again!

With the ability to have a custom HTTP code returned when a mock isn't found, I found that it would also be useful to reset mocks, and return the `SimpleHttpResponseProvider` back to its original state. I've added a simple `reset()` method that does this.

I did think about adding this to the `HttpResponseProvider` interface, but that seems very simple. And resetting the mocks for something like the `FileHttpResponseProvider` didn't make much sense.

Let me know what you think.