Fixed issue where 'is_exclusive' is being used by surveyor but not surveyor-ios on answers. Bug #3464

Based on the surveys in the surveyor_ios project, the answer's exclusivity was based on the key "exclusive". It turns out that "exclusive" and "is_exclusive" are used in tandem due to a bug with surveyor. In order to safeguard against this issue, surveyor_ios now checks against both.