Fix Authentication token expiration check

[tonglil]

Closes #184 and #190.

Throw an error if a token does not exist when building the Authorization header.

When using the GitHub OAuth app to authenticate, this is the secure object:

secure {
  tokens: {
    access_token: 'ghu_xxxxxxxxxxxxxxxxxxx',
    token_type: 'bearer',
    scope: ''
  },
  expires_at: null
}

expires_at is null, so it always calls buildHeaders with an empty string (which doesn't make sense).

Removed this erroneous code and throw an error if the header is being built without a token.

[github-sebastien-boulet]

👍

[karpikpl]

Thanks for the PR, let me review and test.
I never run it with OAuth app