code: public String putHeader(final String name, final String value) {
          if (headers == null) {
              headers = new TreeMap<>(String.CASE_INSENSITIVE_ORDER);
          }
          return headers.put(name, value);
      }
docs: Associates the specified header value with the specified name.
  
  @param name
  of the header
  @param value
  of the header
  
  @return the previous value associated with the <tt>name</tt>, or
  <tt>null</tt> if there was no mapping for <tt>name</tt>.