[3.1.1 backport] /db/ routing does not work with default collection that coexists with named collection

The API routing we use to allow /db/ to map to /db._default._default/ doesn't work when the default collection is explicitly defined alongside a named collection.

• Removing the named collection makes this work (i.e. we're handling an explicit default collection OK, but only when it's the ONLY collection defined).

Repro steps:

1. Create database with _default scope, the _default collection and a named collection
2. Make a doc using PUT /db._default._default/doc1 and it works
3. Make a doc using PUT /db._default.customCollection/doc2 and it works
4. Make a doc using PUT /db/doc3 and get a 404 keyspace not found error
   • This should route to the default collection, like in step 2

Skipped unit test that repros: TestCollectionsPutDocInDefaultCollectionWithNamedCollections