Improve output cache.
Merge branch 'develop' of https://github.com/EllisLab/CodeIgniter into feature/output-cache-improvements
Replace is_null() with === / !== NULL

Exact same behavior, but faster. I also think it's more readable.
[ci skip] Fix issue #2157 - docs on the email library
A few adjustments to previous commit
Merge pull request #2152 from vlakoff/develop

Replace is_null() with === / !== NULL
Merge branch 'develop' of https://github.com/EllisLab/CodeIgniter into feature/output-cache-improvements
Fix issues #2160, #2161
Return spacing on var definitions.

Signed-off-by: Eric Roberts <eric@cryode.com>
Merge pull request #2151 from cryode/feature/output-cache-improvements

Add headers to Output class cache files.