Override handleOAuthAccessTokenResponse to parse Slack's OAuth2 v2 token response

[jonstorer]

Please see https://github.com/jaredhanson/passport-oauth2/pull/174 AND https://github.com/nmaves/passport-slack-oauth2/issues/9

Slack's OAuth2 v2 implementation overloads the OAuthTokenResponse json payload to return multiple tokens. The root of the json object is for bot tokens & user tokens exist in a property called authed_user.

Passport is designed to authenticate Users. When passport attempts to use the root level accessToken, it either does not exist (no bot scopes provided in authorization request) or is a bot token.

This change leverages a new hook in the passport-oauth2 library to reformat the OAuthTokenResponse payload to set the correct accessToken, refreshToken, and params for the following scenarios:

• user
• user and bot
• bot

---

The profileUrl is no longer defaulted during configuration. When the profile is not being skipped & a custom profileUrl was not provided, the profileUrl is set during handleOAuthAccessTokenResponse depending on which token is the root of the params object.

---

if this is merged, the README should be updated to emphasize the verify callback that accepts the params object from the OAuthTokenResponse request.

the params object will be reformatted to:

user_scope - user only

{
  id: 'user-id',
  scope: 'user-scope-1,user-scope-2',
  token_type: 'user',
  access_token: 'user-access-token',
  refresh_token: undefined,
  authed_user: {
    id: 'user-id',
    scope: 'user-scope-1,user-scope-2',
    access_token: 'user-access-token',
    token_type: 'user'
  },
  authed_bot: {}
}

scope - bot only

{
  id: 'bot-user-id',
  scope: 'bot-scope-1,bot-scope-2',
  token_type: 'bot',
  access_token: 'bot-token',
  refresh_token: undefined,
  authed_user: { id: 'user-id' },
  authed_bot: {
    id: 'bot-user-id',
    scope: 'bot-scope-1,bot-scope-2',
    token_type: 'bot',
    access_token: 'bot-token',
    refresh_token: undefined
  }
}

scope & user_scope - bot & user

{
  id: 'user-id',
  scope: 'user-scope-1,user-scope-2',
  token_type: 'user',
  access_token: 'user-access-token',
  refresh_token: undefined,
  authed_user: {
    id: 'user-id',
    scope: 'user-scope-1,user-scope-2',
    access_token: 'user-access-token',
    token_type: 'user'
  },
  authed_bot: {
    id: 'bot-user-id',
    scope: 'bot-scope-1,bot-scope-2',
    token_type: 'bot',
    access_token: 'bot-token',
    refresh_token: undefined
  }
}