Changed toIso8601String helper function

[afrojuju1]

Changed toIso8601String to take in a boolean which when false would remove the timezone from ISO8601 string.
This adds extra flexibility around ISO8601 string format.

[Glavic]

If you need custom format, why don't you use ->format() method?

[kylekatarnls]

Each ->toX method should be reserved for one format. I recommend you to extend the class (or use a macro if you work with Laravel) to create an other method for your custom format.

[briannesbitt]

We would implement this as a separate method anyway, no boolean param. But regardless, doesn't need to go into Carbon. Sounds like a good place for your own class.

[kylekatarnls]

Will appear in the documentation:
gh-pages...kylekatarnls:gh-pages-1.26