title: OKHttp Add header only if it doesn't already exist
body: <p>I couldn't find a way to add an header only in case it doesn't exist.
  What I actually need is to add the default header
  <code>&quot;Content-Type&quot;: &quot;application/json&quot;</code>
  but only in case the header doesn't exist already.
  One way to do it would be to have a different Http client or a different interceptor for when the default is necessary but I was expecting to be able to check whether the header is already there and add it only in case it is not.</p>