Issue with double prefixed shadow tables

CakePHP adds a prefix to the table name. But `$fullTableName` already includes this prefix leading to double prefixed shadow tables. `$fullTableName` is still used for a check of the existence of the table.

So `<prefix><prefix><table>_revs` was used for saving revisions but `<prefix><table>_revs` is checked for existence. To use this behavior, you had to create both of them.

The documentation states you only need to create `<prefix><table>_revs`. This commit resolves this problem.