Cleaner URL for paginatorHelper

Avoid appending &sort and &direction if the sort parameters are the one from options

## Problem

PaginatorHelper is appending **sort** and **direction** parameter even if it's not needed

## Solution

Pass the default **sort** and **direction** into $this->request->params['paging'] (sortDefault and directionDefault) 
Within the PaginatorHelper::url() method check if the params are the same and unset $url['sort'] and $url['direction']

## Use Case

    // Controller
    public $paginate =  ['order' => ['Posts.created' => 'asc'], 'limit' => 10]; 

    // View 
    $this->Paginator->numbers()
    // Was generating 
    url?page=2&sort=Posts.created&direction=asc
    // Now generates
    url?page=2

If more than one order is defined my behavior is not used and the sort parameter is injected again 

    // Controller
    public $paginate =  ['order' => ['Posts.created' => 'asc', 'Posts.name' => 'asc'], 'limit' => 10]; 

    // View 
    $this->Paginator->numbers()
    // Was generating and will keep generating this
    url?page=2&sort=Pots.created&direction=asc

That's still a problem but (since the default order will be lost) but in my opinion it's a specific use case.
I don't know if paginator is supposed to support multiple orders so I didn't edited this use case.