http.CloseNotifier is deprecated

[wayneashleyberry]

middleware/wrap_writer.go:103:38: http.CloseNotifier is deprecated: the CloseNotifier interface predates Go's context package. New code should use Request.Context instead.  (megacheck)
	cn := f.basicWriter.ResponseWriter.(http.CloseNotifier)
	                                    ^
middleware/wrap_writer.go:129:7: http.CloseNotifier is deprecated: the CloseNotifier interface predates Go's context package. New code should use Request.Context instead.  (megacheck)
var _ http.CloseNotifier = &httpFancyWriter{}

Started seeing this pop up in my linters, not sure if this code could simply be removed?

• chi/middleware/wrap_writer.go

  Line 103 in b5294d1

  cn := f.basicWriter.ResponseWriter.(http.CloseNotifier)

• chi/middleware/wrap_writer.go

  Line 143 in b5294d1

  cn := f.basicWriter.ResponseWriter.(http.CloseNotifier)

---

From godoc.org:

Deprecated: the CloseNotifier interface predates Go's context package. New code should use Request.Context instead. — https://golang.org/pkg/net/http/#CloseNotifier

[pkieltyka]

done in #378