code: public void put(String key, String value)
      {
          put(key, value, DefaultCharset.get());
      }
docs: Set a user metadata entry.
  <p>
  This method and its {@link RiakUserMetadata#get(java.lang.String) }
  counterpart use the default {@code Charset} to convert the {@code String}s.
  </p>
  
  @param key   the key for the user metadata entry as a {@code String} encoded using the default {@code Charset}
  @param value the value for the entry as a {@code String} encoded using the default {@code Charset}