(#3195) - Stringify "since" parameter when fetching _changes

In CouchDB 2.0, the sequence number may be represented as an array. This PR stringifies the "since" parameter if it is an object to cope with this scenario.

I'm assuming that the sequence number will be represented as JavaScript object throughout PouchDB and only serialized to a string when required. There is obviously further work to do to ensure that we're treating sequence numbers appropriately in other parts of the code (e.g. comparison operators) - this is just the first issue I came across during testing.
nit `if (typeof opts.since`