Update CI matrix to go1.10 + go1.11 and fix ResponseWriter bugs

Closes #186 (closed)

Go 1.9 is no longer supported so we remove that from CI.

Go 1.11 was failing in CI. The reason, it turned out, is that we had several incorrect implementations of io.Writer in our code. These were causing errors in httputil.ReverseProxy all along, but until go 1.11, these errors were ignored. Since go 1.11 they cause the entire request to be aborted.

This MR fixes the broken writers and adds tests.