Don't set content-length when serving assets.

Setting content-length on assets causes long pauses when fetching assets
that contain PHP code. These assets are invariably larger than their
response bodies. By not setting a content-length, we can allow the
webserver to calculate it for us.

Refs #4916
Merge pull request #5513 from cakephp/issue-4916

Don't set content-length when serving assets.