Stop relying on client-side docType in findAll reduce function

Another fix without tests, sorry. I couldn’t get the tests working with a CouchDB instance.

The reduce function in `findAll` isn’t working when PouchDB defers to a CouchDB instance because the `findByDocType` function relies on a client-side variable. I used an `eval` to construct a self-contained `findByDocType`, but maybe there’s a better way?