title: woocommerce api returning html, not json
body: <p>I have a test site which calls are returning in JSON no problems,</p>
  
  <p>But on my live site (different from test site), it is returning html.</p>
  
  <p>I have also put in the header "Content-Type: application/json" but it is still returning HTML.</p>
  
  <p>I have:</p>
  
  <p>W3 Total Cache
  and woo commerce Version 2.6.14 installed.</p>
  
  <p>I know it is an old version and I am upgrading but there is alot to sort out before upgrading, but this shouldn't cause this issue should it?</p>
  
  <p>I am calling on postman.</p>
  
  <p>Why is HTML being returned on API Calls and how do I fix it?</p>