report.JSON breaks websockets

[ohookins]

If you wrap your websocket Handler in report.JSON you end up with something like this:

2017/01/10 14:15:43 websocket: response does not implement http.Hijacker

[telendt]

@ohookins: That's quite common problem, not only present in handy but in most libs wrapping various "upgradable" interfaces.
Here's a good read about upgradeable interfaces, the problems they solve and the ones they bring:
https://avtok.com/2014/11/05/interface-upgrades.html

This PR solves it for gzipWriter. But there are more interfaces which ResponseWriter can be upgraded to...

[ohookins]

@telendt I guess that's just a simpler version of this:
https://github.com/prometheus/client_golang/blob/master/prometheus/http.go#L296-L306

[ohookins]

@streadway are you ok with me making a PR for report along the lines of #28 ? Or perhaps @telendt is kind enough to add report to the same PR...

[streadway]

Go for it. There are a few more interfaces to implement as well for http/2 support.