strtotime needs a special case

[bendavies]

Hi there!
Thanks for the great library!
I think I've found a bug when integrating safe into my project.

Because of this:
https://3v4l.org/HL9Xn
strtotime('+1 day') is not equivalent to \Safe\strtotime('+1 day'), so looks to need some special handling, to not pass null as the 2nd param if the caller didn't specifically pass null, or enforce int for the 2nd param, or pass time() manually if null.

Thanks

[moufmouf]

True! The default value for the second parameter in the PHP doc is "time()" (which of course is not a "valid" default value from a PHP standpoint). It gets replaced by null and that causes the issue.

Looking at the documentation, the touch function might also be impacted.

[moufmouf]

Closed in #85