Form helper _attributes_to_string() micro-optimization

As $attributes should be most of the times an array, let's save an is_string() call.
Polishing Form helper
Change spaces to tabs
Fix #2568
Merge pull request #2567 from vlakoff/develop-2

Polishing Form helper
Cache Log Error Levels
The log messages in the cache drvier's is_supported calls are more suited for the debug level.
Updated Changelog Cache Log Level
- Updated the changelog to include the log level updates in the cache drivers.
Merge pull request #2569 from refringe/develop

Cache Log Error Levels
Optimize CI_Session::__construct() routines and make driver validity check stricter
Add windows filename rule as an option for upload files
Revert "Add windows filename rule as an option for upload files"

This reverts commit 23719ab569c9c8d6b791f65d7861daba3895ddcb.
Make the bad filename array public in Security library
Rename bad chars property to filename_bad_chars, remove the setter and add changelog entry
Merge pull request #2575 from wmh/filename_rule

Add windows filename rule as an option for upload files
[ci skip] Alter a changelog line
Merge pull request #2566 from vlakoff/develop

Complete 3.0 upgrade notes about NULL returned on missing items
Fix #2501 & another -related bug
Fix #2585