Update the table cache in update_table (fixes #67)

The problem occurs in the following scenario:

* an instance of a Table, obtained through Database.get_table, is used
  to insert some data
* a different instance of Table, obtained through Database.get_table
  call after the data has been inserted, uses a Table instance that is
  not up-to-date

The result is that, e.g. the keys created with the first Table object
aren't accessible via the second object.

This fixes the problem by updating the table cache in
Database.update_table.