Fix /instantws handler (Thanks jwilk)

Fix /instantws handler (Thanks jwilk)

fixes #84

This was broken with commit c4e4211c0a3ba958e2a973ddecd38126d40e1f02.

Stack trace:

Oct 01 07:44:57 dcs-instant-web dcs-web[31174]: 2017/10/01 07:44:57 server.go:2848: http: panic serving [::1]:33090: interface conversion: *nethttp.statusCodeTracker is not http.Hijacker: missing method Hijack
Oct 01 07:44:57 dcs-instant-web dcs-web[31174]: goroutine 1287698 [running]:
Oct 01 07:44:57 dcs-instant-web dcs-web[31174]: net/http.(*conn).serve.func1(0xc4201726e0)
Oct 01 07:44:57 dcs-instant-web dcs-web[31174]:         /home/michael/sdk/go1.9/src/net/http/server.go:1697 +0xd0
Oct 01 07:44:57 dcs-instant-web dcs-web[31174]: panic(0x91dbc0, 0xc4a0a5b080)
Oct 01 07:44:57 dcs-instant-web dcs-web[31174]:         /home/michael/sdk/go1.9/src/runtime/panic.go:491 +0x283
Oct 01 07:44:57 dcs-instant-web dcs-web[31174]: github.com/Debian/dcs/vendor/golang.org/x/net/websocket.Server.serveWebSocket(0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
Oct 01 07:44:57 dcs-instant-web dcs-web[31174]:         /home/michael/go/src/github.com/Debian/dcs/vendor/golang.org/x/net/websocket/server.go:74 +0x91
Oct 01 07:44:57 dcs-instant-web dcs-web[31174]: github.com/Debian/dcs/vendor/golang.org/x/net/websocket.Handler.ServeHTTP(0x9dfb90, 0xe96f40, 0xc421bd45a0, 0xc4201aa100)
Oct 01 07:44:57 dcs-instant-web dcs-web[31174]:         /home/michael/go/src/github.com/Debian/dcs/vendor/golang.org/x/net/websocket/server.go:112 +0xb4
Oct 01 07:44:57 dcs-instant-web dcs-web[31174]: net/http.(*ServeMux).ServeHTTP(0xc420232cc0, 0xe96f40, 0xc421bd45a0, 0xc4201aa100)
Oct 01 07:44:57 dcs-instant-web dcs-web[31174]:         /home/michael/sdk/go1.9/src/net/http/server.go:2254 +0x130
Oct 01 07:44:57 dcs-instant-web dcs-web[31174]: github.com/Debian/dcs/vendor/github.com/opentracing-contrib/go-stdlib/nethttp.Middleware.func2(0xe97b40, 0xc420323180, 0xc421610800)
Oct 01 07:44:57 dcs-instant-web dcs-web[31174]:         /home/michael/go/src/github.com/Debian/dcs/vendor/github.com/opentracing-contrib/go-stdlib/nethttp/server.go:90 +0x3c6
Oct 01 07:44:57 dcs-instant-web dcs-web[31174]: net/http.HandlerFunc.ServeHTTP(0xc420232d20, 0xe97b40, 0xc420323180, 0xc421610800)
Oct 01 07:44:57 dcs-instant-web dcs-web[31174]:         /home/michael/sdk/go1.9/src/net/http/server.go:1918 +0x44
Oct 01 07:44:57 dcs-instant-web dcs-web[31174]: net/http.(*ServeMux).ServeHTTP(0xecebc0, 0xe97b40, 0xc420323180, 0xc421610800)
Oct 01 07:44:57 dcs-instant-web dcs-web[31174]:         /home/michael/sdk/go1.9/src/net/http/server.go:2254 +0x130
Oct 01 07:44:57 dcs-instant-web dcs-web[31174]: net/http.serverHandler.ServeHTTP(0xc420234a90, 0xe97b40, 0xc420323180, 0xc421610800)
Oct 01 07:44:57 dcs-instant-web dcs-web[31174]:         /home/michael/sdk/go1.9/src/net/http/server.go:2619 +0xb4
Oct 01 07:44:57 dcs-instant-web dcs-web[31174]: net/http.(*conn).serve(0xc4201726e0, 0xe98680, 0xc4a0a5a580)
Oct 01 07:44:57 dcs-instant-web dcs-web[31174]:         /home/michael/sdk/go1.9/src/net/http/server.go:1801 +0x71d
Oct 01 07:44:57 dcs-instant-web dcs-web[31174]: created by net/http.(*Server).Serve
Oct 01 07:44:57 dcs-instant-web dcs-web[31174]:         /home/michael/sdk/go1.9/src/net/http/server.go:2720 +0x288