Simple fix for protocol buffer RpbBucketProps

In Riak 2.0.0 pre5, I've noticed that the repl (replication) flag can be null. The way protocol buffer's generated the C# code does not correctly handle this case. Here's a really simple, minor fix that works. It may be undesirable given that it sets replication to false by default in the absence of a value but I didn't feel comfortable changing it to anything else given it could have very unexpected side-effects. In this case, the user is required to configure the bucket, but I don't see the harm in that... (he said naively)