performance improvements in Model::_clearCache

- don't empty cache twice if pluralized name is identical to underscored
- refactors for fewer function calls
Refactors Model::_clearCache

- don't use mixed keys in $assoc array
- removes unnecessary strtolower on Inflector::underscore result
- return early
Merge pull request #1687 from Schlaefer/model-cache-improvements

performance improvements in Model::_clearCache