Fixes parsing of multiple push messages in streaming mode (#12197)

The current atmosphere code can handle messages split up using the message
length header. But it does not handle the case where the header is actually
split up itself.
This patch solves it by making sure the length header stays intact.
Corresponding atmosphere ticket:
https://github.com/Atmosphere/atmosphere/issues/1199

Change-Id: If22c42ea63b0749c68df19c869b316f9b69f4317