QuestionFragment: Fix a typo to really save the checkbox selections.
ItemsContentProvider: Add the checkbox selections to the POST too.

However, HttpParams.setParameter() does not support multiple parameters
with the same name, as expected by the Galaxy-Zoo server. For instance:
classification[annotations][9][sloan-6]=a-0
classification[annotations][9][sloan-6]=x-2
classification[annotations][9][sloan-6]=x-3
if the user selected two checkboxes (x-2) and (x-3) for the
"Is there anything Odd?"question and then clicked Done (a-0).