Specifying a negative receive expectation could be considered to break respond_to?

Specifying a negative receive expectation (`expect(...).to_not receive(:blah)`) could be considered to break `respond_to?`, as reported via twitter:

[![screen shot 2014-08-21 at 17 00 37](https://cloud.githubusercontent.com/assets/162976/3998951/5a6a2c22-294c-11e4-99e2-e5185f1ad5e9.png)
](https://twitter.com/damncabbage/status/502367823822213120)


I'm on the fence by this, it's slightly surprising but not unexpected given a knowledge of the internals, the fix is complicated because we can't just return false from `respond_to?` for negative expectations (that could be a lie).