code: public StreamEx<String> join(CharSequence delimiter) {
          return map(entry -> String.valueOf(entry.getKey()) + delimiter + entry.getValue());
      }
docs: Returns a {@link StreamEx} of strings which are created joining the keys
  and values of the current stream using the specified delimiter.
  
  <p>
  This is an <a href="package-summary.html#StreamOps">intermediate</a>
  operation.
  
  @param delimiter the delimiter to be used between key and value
  @return the new stream
  @since 0.2.2