Re-instate the capture mode of View::append().

Use the existing features of ViewBlock to create an append() with capture mode.

The discussion in cakephp/docs#1402 showed that there is still a use for a capturing mode to append(). Having a capture mode means that `append('block', null)` will work differently and now enable a capturing block whereas before it would append '' to the block.
This fixes the tests not outputting for the last little bit.