Enable metrics and access logging with gzip turned on

[tino]

Fixes #460.

I wasn't able to find a way to fix this transparently so that casting to *httputil.ReverseProxy would still work (as is done on line 200), so fixed it this way.

I tried to add an combined integration test of TestProxyLogOutput and TestProxyGzipHandler but couldn't get it to fail. I suppose that's got something todo with my go-testing skills :D.

[magiconair]

Yes, that will fix it but I think there are two better alternatives:

1. Wrap the response writer to capture the status code and the content length since these are the only two values the logger needs
2. wrap the transport in the proxy handler instead of the http_handler.go function. Then there is no need to cast and type check.

[magiconair]

I'll have a quick look

[magiconair]

I've created #470 for option 1 and added a test. I think that's a cleaner way of solving this. Again, thanks for tracking this down. Could you test #470, please?