Check for bucket arguments more loosely

It allows to inherit from Couchbase::Bucket

Change-Id: Ib4577b1500734b2f9e15aaa4cb4b35e961faceac
Reviewed-on: http://review.couchbase.org/32331
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>