code: public void put(String key, String value, Charset charset)
      {
          BinaryValue wrappedKey = BinaryValue.unsafeCreate(key.getBytes(charset));
          BinaryValue wrappedValue = BinaryValue.unsafeCreate(value.getBytes(charset));
          meta.put(wrappedKey, wrappedValue);
      }
docs: Set a user metadata entry.
  <p>
  This method and its {@link RiakUserMetadata#get(java.lang.String, java.nio.charset.Charset)  }
  counterpart use the supplied {@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 supplied {@code Charset}
  @param value the value for the entry as a {@code String} encoded using the supplied {@code Charset}