Bucket#cas doesn't set original flags

The Bucket#cas operation resets flags

  > Couchbase.bucket.set("foo", "bar", :flags => 0x100)
  17982951084586893312
  > Couchbase.bucket.cas("foo") { "baz" }
  1712422461213442048
  > Couchbase.bucket.get("foo", :extended => true)
  ["baz", 0, 1712422461213442048]

So it could be dangerous if someone using custom flags or non-default value encoding (not :document)