api: check before using pre-resolved address

[axw]

Description of change

When dialing a websocket, check that the address
we're asked to dial is the same as the one that
we pre-resolved. It will not match when we're
using a proxy.

This is a short-term fix. If we should be doing
any client-side resolution at all, it should be
disabled when a proxy is to be used.

QA steps

1. juju bootstrap localhost
2. go get github.com/elazarl/goproxy, then go run the following program:

package main

import (
    "github.com/elazarl/goproxy"
    "log"
    "net/http"
)

func main() {
    proxy := goproxy.NewProxyHttpServer()
    proxy.Verbose = true
    log.Fatal(http.ListenAndServe(":8080", proxy))
}

3. export https_proxy=localhost:8080
4. juju status

Documentation changes

None.

Bug reference

Fixes https://bugs.launchpad.net/juju/+bug/1698989

[axw]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju