This reintroduces support for Go modules, as v4.
CloseNotifier() is removed as it has been obsoleted, see https://golang.org/doc/go1.11#net/http
It was already NOT working (not sending signals) as of 1.11 the functionality was gone, we merely
deleted the functions that exposed it. If anyone still relies on it they should migrate to using
`c.Request().Context().Done()` instead.
Closes#1268, #1255
