Pubsub message size limit not taking size increase from base64 encoding into account

In the PubSubIO, the default max batch size is set to `10 * 1024 * 1024` bytes. This however does not take into account the size increase of base64 encoding the messages after the flush. Base64 encodes each set of three bytes into four bytes.

Therefore the 'true' size limit placed on the unencoded batch should be
x = ((10 * 1024 * 1024) / 4) * 3 = 7864320 bytes