[5.6] Add sortKeys and sortKeysDesc methods to Collection.

[pmatseykanets]

This PR adds two convenience methods to Collection to allow sorting keys of the collection, or sorting the collection by the key if you wish.

• sortKeys
• sortKeysDesc

One of the use cases that I find myself a lot lately is grabbing an object via an API and rendering it's properties in a sorted fashion.

This PR allows to do something along the lines of

$this->renderLead(collect($this->getLeadViaAPI($id))->sortKeys());

instead of

$leadData = get_object_vars($this->getLeadViaAPI($id));
ksort($leadData);

$this->renderLead(collect($leadData));

or doing some unnatural combination of map and sort.