title: Request header not set as expected when using 'no-cors' mode with fetch API
body: <p>I have a fetch where the request types seems to be changing which is messing up my post.  I submit my basic form (one field only).  Here is the fetch.  </p>
  
  <pre><code>      handleSubmit(event, data) {
      //alert('A name was submitted: ' + this.state.value);
      event.preventDefault();
      console.log("SUBMIT STATE::", this.state.value);
      return (
          fetch("//localhost:5000/api/values/dui/", {
              method: "post",
              mode: 'no-cors',
              headers: {
                  'Access-Control-Allow-Origin': '*',
                  'Content-Type': 'application/json',
                  'Accept': 'application/json',                  
              },
              body: JSON.stringify({
                  name: this.state.value,
              })
          }).then(response =&gt; {
              if (response.status &gt;= 400) {
                  this.setState({
                      value: 'no greeting - status &gt; 400'
                  });
                  throw new Error('no greeting - throw');
              }
              return response.text()
          }).then(data =&gt; {
              var myData = JSON.parse(data);
              this.setState({
                  greeting: myData.name,
                  path: myData.link
              });
          }).catch(() =&gt; {
              this.setState({
                  value: 'no greeting - cb catch'
              })
          })
      );
  
  
  }
  </code></pre>
  
  <p>But when I look at this in fiddler content-type is now 'content-type: text/plain;charset=UTF-8'.  Here is the raw Fiddler:</p>
  
  <pre><code>POST http://localhost:5000/api/values/dui/ HTTP/1.1
  Host: localhost:5000
  Connection: keep-alive
  Content-Length: 16
  accept: application/json
  Origin: http://evil.com/
  User-Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/56.0.2924.87 Safari/537.36
  </code></pre>
  
  <p>content-type: text/plain;charset=UTF-8
     Referer: <a href="http://localhost:3000/" rel="nofollow noreferrer">http://localhost:3000/</a>
     Accept-Encoding: gzip, deflate, br
     Accept-Language: en-US,en;q=0.8</p>
  
  <pre><code>{"name":"molly"}
  </code></pre>
  
  <p>In DOM Inspector I just see:</p>
  
  <p>POST <a href="http://localhost:5000/api/values/dui/" rel="nofollow noreferrer">http://localhost:5000/api/values/dui/</a> 415 (Unsupported Media Type)</p>
  
  <p>I also find it strange that 'accept' is lower case as well as 'content-type'.  Any reason why this is happening.  I haven't found anything specific in my searches yet.    </p>