Update: If there is no sorting methods for the column type, use string

- In the case of filtering, if there is no filtering extension for the
  column type, then the basic string based filter formatting is
  performed (i.e. string is always the fallback - there can't be an
  error because a function isn't there).

- This commit matches that behaviour in sorting. If you define a column
  type as something for which there is no column sorting function, then
  the string type will be used rather than resulting in a javascript
  error.