Add failing test: TTL=0 means "never expire"

File driver fails this test because
its implementation for TTL=0 is "expire now"
Fix Cache_File TTL=0 to mean "never expires"

The cache expiration test is now negative when
`$lifetime` (TTL) is 0.

Closes Github issue #50
Merge pull request #62 from kohana/3.3/bugfix/ttl-0-means-never-expires

Fix Cache_File TTL=0 to mean "never expires"