Micro-optimization of JDocumentRendererHead/render

The current code forJDocumentRendererHead/render uses an output buffer to return the result of a single echo statement. While very cool, I don't belive there is any advantage to this other than demonstrating excellent usage of output buffering.
With this change, my tests shows that we can expect the head portion of the page to render up to 0.000009 seconds faster while consuming up to 28,184 bytes less memory ;-) It also reduces the method to a single return statement.

Tracker item http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_id=8103&tracker_item_id=33534