Can.string getNext function fix

Fix getNext function in can.string component
There was wrong logic behind evaluation if add or not add empty object

```javascript
// Before
getNext = function( obj, prop, add ) {
			return prop in obj ?
				obj[ prop ] : 
				( add && ( obj[ prop ] = {} ));
		}

getNext({}, 'foo', undefined) // -> undefined // this is ok
getNext({}, 'foo', false) // -> false // instead of undefined
getNext({}, 'foo', "") // -> "" //instead of undefined
getNext({}, 'foo', true) // -> {} // which is OK
etc....

// ------------------
//After
getNext = function( obj, prop, add ) {
      var result = obj[prop];

      if(result === undefined && add === true) { result = obj[prop] = {} }
      return result
    }

getNext({}, 'foo', undefined) // -> undefined
getNext({}, 'foo', false) // -> undefined
getNext({}, 'foo', "") // -> undefined
var obj = {}
getNext(obj, 'foo', true) // -> {} // and correctly obj = {foo: {}} 
```