title: requests.post only accepts "text" dictionary values
body: <p>I'm attempting to make post requests to slack from a requests.post() call.  Here is an example of the code that I've made to do this:</p>
  <pre><code>    fields = {
          'sender': message['From'],
          'subject': message['Subject'],
      }
  
      url = 'https://hooks.slack.com/services/XXXXXX'
      print(fields)
      return requests.post(url, json=fields)
  </code></pre>
  <p>The issue seems to be that the <code>requests.post(url, json=fields)</code> command only seems to respond to the fields command if I change the key value(s) to &quot;text&quot;.  I have no idea what is not allowing the requests.post command to use other key values (such as sender or subject) that I have defined here.</p>
  <p>I have been able to fix this by doing a terrible workaround of calling two separate requests.post commands and setting sender/subject as their own variables with json formatted dictionaries:</p>
  <pre><code>sender = {'text': message['From']}
  subject = {'text': message['Subject']}
  </code></pre>
  <p>Just a side-note I'm gathering these message handlers from aiosmtpd.handlers which allows me to parse smtp messages that come in.</p>