Leave db->cacheSources unaltered.

When a model uses cacheSources = false, it should not have side effects
on the datasource.

Fixes #2364
Only enable cacheSources if both the datasource + model agree on it.

This prevents models from flipping cacheSources on when the datasource
has it disabled already.

Refs #2364
Fix coding standards error.

Refs #2364
Merge pull request #2367 from cakephp/issue-2364

Leave db->cacheSources unaltered.

Fixes #2364