Twitter API with iron-ajax?

Question

I have an iron-ajax element set up:

<iron-ajax
  id="get-tweets"
  url="http://api.twitter.com/1.1/search/tweets.json?q=nasa&result_type=popular"
  handle-as="json"
  on-response="handleResponse"
  debounce-duration="300">
</iron-ajax>

And I'm trying to pass an authentication header to it for Twitter, but I can't figure out how. Twitter has an example request and I'm trying to figure out how to pass the header to the ajax request:

getTweets() {
  const ajax = this.root.querySelector('#get-tweets');
  ajax.headers = Object.assign(ajax.headers, `what do I put here?`
  });
  ajax.generateRequest();
},

I can't seem to figure out how to do it. Let's say I have the following:

Consumer API keys:
abc (API key)
def (API secret key)
Access token & access token secret:
gh-ijk (Access token)
lmn (Access token secret)
Read and write (Access level)

How do I pass this info to the iron-ajax element so I can make the request?