Add Inflector::hyphenate().
Return null instead of false on cache miss.

Null better represents "no value found".
Update "static" keyword instead of class name.
Improve arugement name and docblock.
Add tests for null and false input to inflector methods.
Use static:: instead of Inflector::, missed one earlier.
Rename Inflector::hyphenate() to Inflector::dasherized()
Revert to returning false on cache miss.
Merge pull request #4460 from cakephp/3.0-inflector

3.0 inflector