Validate numeric argument to _revs_limit.
Validate numeric argument to _revs_limit.

Closes COUCHDB-1087

Patch by Lukasz Mielicki.

apache/couchdb-couch,apache/couchdb-couch,fkaempfer/couchdb,ngpestelos/couchdb-couch,apache/couchdb-couch,ngpestelos/couchdb-couch,cloudant/couchdb-couch,alanning/couchdb-couch,alanning/couchdb-couch,KlausTrainer/couchdb-couch,cloudant/couchdb-couch,cloudant/couchdb-couch,smithsz/couchdb-couch,fkaempfer/couchdb,alanning/couchdb-couch,fkaempfer/couchdb,fkaempfer/couchdb,KlausTrainer/couchdb-couch,smithsz/couchdb-couch,ngpestelos/couchdb-couch,KlausTrainer/couchdb-couch,fkaempfer/couchdb,smithsz/couchdb-couch,fkaempfer/couchdb