(daleharvey/pouchdb#1198) - Allow 0/null as mapreduce keys

CouchDB allows 0 and null as separate keys in
mapreduce queries. This commit makes PouchDB behave like CouchDB.
Since this is a multi-repo change that affects pouchdb,
pouchdb-collate, and pouchdb-mapreduce, I wasn't really sure
how to structure this commit, so I'm just including the same
commit message in 3 separate commits.