[writer] Prevent adding unnecessary space character to end of a line within block collections.

There are no **real** problems with that, but looks not nice:

``` javascript
var data = { data: [ 'foo', 'bar', 'baz' ] };

console.log('%j', yaml.dump(data));

// Output: "data: \n  - foo\n  - bar\n  - baz\n"
// Note an odd space after "data:".
```
