code: public OkHttpRequest addHeader(String name, String value) {
          builder.addHeader(name, value);
          return this;
      }
docs: Adds a header with {@code name} and {@code value}. Prefer this method for multiply-valued
  headers like "Cookie".
  
  <p>Note that for some headers including {@code Content-Length} and {@code Content-Encoding},
  OkHttp may replace {@code value} with a header derived from the request body.