fix the node-fetch cloned response hanging bug as best I can

[wheresrhys]

While this is technically a breaking change - responses returned by inspected calls won't always be readable in the way they were before - I feel it's necessary to roll back the hanging bug for users of v9. Prioritising the users of this new feature (only released about a month ago) over established users who may come across this troubling bug #575 feels like poor prioritisation.

The solution I've gone with is to clone responses on demand in the inspection stage. If this fails (because the response has already been read), it falls back to overwriting the json, text etc methods of the original response with functions that return the promises generated by those original function calls, which are captured by the proxy wrapped around the original response.

A better solution making more use of the proxy feels possible, along the lines of lazily cloning only at the point that the original body is used. That feels more robust, but also hard work to implement properly, so want to get this fix out first.

[coveralls]

Coverage increased (+0.04%) to 96.4% when pulling e0b574f on nasty-clone-bug into 684a0b4 on master.