JMicrodata: small bug fix, reset params after the display() method

Patch for a small bug.

Problem:
Not all params are reseted after the display() method.
For example:
```php
$microdata = new JMicrodata('Article');
$html = $microdata->enable(false)->property('name')->content('TITLE ARTICLE ')->display()
	. $microdata->property('aggregateRating')->display();
```
It returns
```html
TITLE ARTICLE TITLE ARTICLE
```
but the output should be
```html
TITLE ARTICLE
```
The problem is that the ```$content``` param is not reseted when JMicrodata is disabled.

Bug Tracker: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33454&start=0