Fix PB message keys when field position > 15

The problem occurred while setting 'RpbBucketProps' for any field following `optional uint32 r = 15`, so setting 'w', 'pw', 'dw' will all fail with parsing error on Riak server

According to PB spec: 'Each key in the streamed message is a varint with the value (field_number << 3) | wire_type' so this patch correctly writes varint key to `bytes` instead of a integer.