Avoid decorating COUNT statements.

Because they only return a number, the decorators will fail expecting
non existent keys.

Fixes https://github.com/cakephp/cakephp/issues/2947
Tests that it is possible to count results containing hasMany
associations both hydrating and not hydrating the results.
Merge pull request #2950 from bar/3.0-count

3.0 - Avoid decorating COUNT statements.