Came up with a basic header test...
Also, set the default value of the DataCollection class's $attributes
property to an empty array, so getIterator doesn't freak out
Dispatch now sends the response when finished, optionally.
Tests fixed/updated to reflect this change
Updating the "chunk" method of Response to use the Response's internal
body, instead of the output buffer
Also updated (and fixed) accompanied test
Making HEAD requests clear the response body
Fixing related test