@Override public void setIfModifiedSince(long newValue) {
    super.setIfModifiedSince(newValue);
    if (ifModifiedSince != 0) {
      requestHeaders.set("If-Modified-Since", HttpDate.format(new Date(ifModifiedSince)));
    } else {
      requestHeaders.removeAll("If-Modified-Since");
    }
  }
Returns true if either:
<ul>
  <li>A specific proxy was explicitly configured for this connection.
  <li>The response has already been retrieved, and a proxy was {@link
      java.net.ProxySelector selected} in order to get it.
</ul>

<p><strong>Warning:</strong> This method may return false before attempting
to connect and true afterwards.
gateway/platforms/servlet/src/main/java/io/apiman/gateway/platforms/servlet/connectors/ok/HttpURLConnectionImpl.java