title: CORS issue using axios with Slack API
body: <p>I am using axios, from browser, to interface with Slack webhook API. When sending a post, I try to use</p>
  
  <pre><code>axios.post(url, data)
  </code></pre>
  
  <p>Browser/axios sends an OPTION request to the backend. Included in the OPTION request is </p>
  
  <pre><code>access-control-request-headers:content-type
  </code></pre>
  
  <p>However, Slack's response has</p>
  
  <pre><code>access-control-allow-origin:*
  </code></pre>
  
  <p>but no access-control-allow-headers header. This causes browser/XMLHttpRequest to complain</p>
  
  <pre><code>Request header field Content-Type is not allowed by Access-Control-Allow-Headers in preflight response.
  </code></pre>
  
  <p>Seems like one solution is to tell axios to not send a content-type header in this case, but I can't figure out how to do that.</p>
  
  <p>Thanks.</p>