code: public RestClientOptions putGlobalHeader(String name, String value) {
          globalHeaders.add(name, value);
          return this;
      }
docs: Add a global header which will be appended to every HTTP request.
  The headers defined per request will override this headers.
  
  @param name  The name of the header
  @param value The value of the header
  @return a reference to this so multiple method calls can be chained together