Record IO class should provide a toString(String charset) method

Currently, the toString() function returns the csv format serialized form of the record object.
Unfortunately, all the fields of Buffer type are serialized into hex string. Although this is a loss less conversion, it is not
the most convenient form, when perople use Buffer to store international texts. With
a new function toString(String charset) , the user can pass a charset to indicate the desired way to convert a Buffer to a String.