Cannot empty out activity item data when editing

[johnjamesjacoby]

Steps to reproduce:

1. Edit an activity stream item that would not normally have content. (friend request, new user, etc...)
2. Add some content and save
3. Edit that same item again, and remove the content
4. Save

You cannot empty the content back out again. I think the save() method is ignoring empty values instead of writing them in.

[djpaul]

(In [5513]) Update activity admin edit screen to allow empty values for some fields to be saved. Switch some REQUESTs to POSTs. Fixes #3834