title: Chai: AssertionError: expected [Function] to be a function (when testing exceptions)
body: <p>I'm getting this error <code>AssertionError: expected [Function] to be a function</code> when I'm trying to whether a async function throws an error</p>
  
  <pre><code>it('has invalid password', async () =&gt; {
      const fakeData = { email: userData.email, password: 'something but not the password!.' }
      expect(async () =&gt; { await authService.authenticate(fakeData) }).to.throw(errors.UnauthenticatedError)
  })
  </code></pre>
  
  <p>result:</p>
  
  <pre><code>AssertionError: expected [Function] to be a function
    at Assertion.assertThrows (node_modules/chai/lib/chai/core/assertions.js:1273:32)
    at Assertion.ctx.(anonymous function) (node_modules/chai/lib/chai/utils/addMethod.js:41:25)
    at doAsserterAsyncAndAddThen (node_modules/chai-as-promised/lib/chai-as-promised.js:293:29)
    at Assertion.&lt;anonymous&gt; (node_modules/chai-as-promised/lib/chai-as-promised.js:252:17)
    at Assertion.ctx.(anonymous function) [as throw] (node_modules/chai/lib/chai/utils/overwriteMethod.js:49:33)
    at Context.it (dist/tests/unit/auth-service.spec.js:56:20)
    at Test._mocha2.default.Test.run (node_modules/mocha-http-detect/dist/index.js:84:21)
  </code></pre>
  
  <p>What did I do wrong?</p>