Add use ...\Str to helpers.php
Change code alignment for str_limit function
Let keys() function returns the keys of the Collection items
Merge pull request #4635 from lucasmichot/4.2-string-helper

[4.2] Support : add use ...\Str for string helpers
Added abstract method getDefaultDriver()

The driver() method calls getDefaultDriver(), but nothing in the class says it should be implemented.
Fix punctuation.
Move method.
Merge branch '4.2'