Fix one static analyzer issue and three warnings

Hi Jens,

I just cleaned a couple issues I found:

Static analyzer issue:

    /CouchCocoa/Couch/CouchQuery.m:114:9: Value argument to 'setObject:forKey:' cannot be nil

Warning:

    implicit conversion from enumeration type 'enum NSJSONReadingOptions' to different enumeration type 'NSJSONWritingOptions' (aka 'enum NSJSONWritingOptions')

-- Tito