fix doc block get_request_header()

This method does not return FALSE as said in doc block, the correct
return is STRING or NULL.
Merge pull request #3262 from adrianorsouza/pr-fix-doc-input-class

fix doc block get_request_header()
config_item() to return NULL instead of FALSE for non-existing items

Close #3001
Close #3232
Related: #3244
Optimize the composer_autoload check
E_DEPRECATED is not available in PHP 5.2
Update a config_item() use case for the new NULL return value
Merge branch 'develop' of git://github.com/EllisLab/CodeIgniter into develop