Update user_agents.php

Added Win 8.1 user agent to the array
Added post-increment for $count

It has been added the missing post-increment for variable $count.
Make CI_Lang::line()'s first parameter mandatory (optional doesn't make sense)
Merge pull request #2768 from porquero/patch-1

Added post-increment for $count
Merge pull request #2753 from SirDre/patch-1

Update user_agents.php
[ci skip] Some spaces & docblock fixes
Make CI_Unit_test::set_test_items()'s parameter mandatory (optional doesn't make sense)
Make CI_Input::set_cookie() and cookie helpers set_cookie(), get_cookie(), delete_cookie()'s first (name) parameter mandatory
Make CI_Form_validation::error()'s first parameter mandatory and add a typehint for CI_Form_validation::error()