Calling mkdir with a NULL $context does not make directory

In the `_make_directory` wrapper, call `mkdir` with
respect to the availability of a passed $context
parameter.

Also removed a `mkdir` call in the `set` method
to use the `_make_directory` method instead.
Merge pull request #60 from kohana/3.3/bug/mkdir-context-null

Calling mkdir with a NULL $context does not make directory

fixes #53
closes #54
closes #59