CachePlugin initial tests.

Not perfect, but provides a start of some tests.
Respect existing Cache-Control headers.

Previously the OoyalaCache subscriber would set the Cache-Control header
no matter what. Now it checks for an existing Cache-Control header and
each directive before adding Cache-Control and each respective directive.
Add test documentation to OoyalaCacheTest class.
Test uncovered an incorrect Cache-Control directive setting.

The test now just sets one directive to make sure the default is still added.