borderRadius getter, specs and consistency

fixes #2545

- fix getter behavior for borderRadius because `border-radius` is buggy in some browsers.(http://jsfiddle.net/9vRwY/)  

For example:

```
# CSS

div {
    border: 5px solid #88A;
    border-radius: 50px;
    height: 200px;
    width: 200px;
}

## JS

element.getStyle('border-radius'); // empty on FF30 and IE11
element.setStyle('border-radius', '20px');
element.getStyle('border-radius'); // now it reads ok == '20px'
```
- make result consistent cross-browser

with this PR `getStyles` always return 4 parameters

- added specs 

With PR code: http://jsfiddle.net/YLQft/
With current code: http://jsfiddle.net/YLQft/1/
return ['borderTopLeftRadius', 'borderTopRightRadius', 'borderBottomRightRadius', 'borderBottomLeftRadius'].map(function(corner){
  return el.style[corner] || '0px';
}).join(' ');