When fetching an RObject with an explicit client, don't use defaultClient.

When fetching an RObject using an explicit client, the implementation would
use defaultClient to fetch the bucket.  Since defaultClient is uninitialized,
this of course panics later on a nil pointer.  Thus switch to using the client
as passed.  Also add to a test to ensure it won't reappear.
Merge pull request #37 from MJDSys/FixGetFromPanic

When fetching an RObject with an explicit client, don't use defaultClient.