code: public HttpClient addHeader(String name, Object value) {
          if (mHeaders == null) {
              mHeaders = new HttpHeaderMap();
          }
          mHeaders.add(name, value);
          return this;
      }
docs: Add a header name-value pair to the request in order for multiple values
  to be specified.
  
  @return 'this', so that addtional calls may be chained together