Refresh the oauth_nonce on redirect (#1573)

[simov]

• Cache the initial oauth options passed to request in _oauth.params
• On subsequent calls to init() use the cached _oauth.params
  to invoke the oauth params generation logic again

Fixes #1573

[marshall007]

@simov I just noticed that since the subsequent calls to init() occur after the logic in lib/redirect.js#L135-L139 if you are redirected to an external host your OAuth credentials will be forwarded.

Here's a failing test case where we start at http://localhost:6767 and get redirected to http://127.0.0.1:6768, but the credentials are still set:

tape('no credentials on external redirect', function(t) {
  var s1 = http.createServer(function (req, res) {
    res.writeHead(302, {location:'http://127.0.0.1:6768'})
    res.end()
  })
  var s2 = http.createServer(function (req, res) {
    res.writeHead(200, {'content-type':'text/plain'})
    res.end()
  })
  s1.listen(6767, function () {
    s2.listen(6768, function () {
      request.get(
        { url: 'http://localhost:6767'
        , oauth:
          { consumer_key: 'consumer_key'
          , consumer_secret: 'consumer_secret'
          , token: 'token'
          , token_secret: 'token_secret'
          }
        }, function (err, res, body) {
          t.equal(err, null)
          t.equal(res.request.headers.Authorization, undefined)
          s1.close(function () {
            s2.close(function () {
              t.end()
            })
          })
        })
    })
  })
})

[simov]

Good catch @marshall007 and thanks for the test 👍

Pushed your test and a fix for it.