Fix unit tests in CouchbaseRequestExecutorTests

There appears to be a regression where a KeyNotFoundException is thrown when the SDK tries to lookup the primary node for a key. The exception is thrown here: https://github.com/couchbase/couchbase-net-client/blob/master/Src/Couchbase/Core/VBucket.cs#L46

The exception is caught, but the non-primary node is then selected as the server the key is mapped to the non-primary. Note this appears to be introduced after 2.5.2 and eyeballing the value used to lookup in the primary in the dictionary - it appears to be the same.