Fix bug with partial append network requests
Our write_functions returned the amount of bytes in our buffer, rather than the amount of bytes we wrote.
This is worked for most cases where the transmission happened in a single block, but when network conditions changed, this caused failures whenever we had to cause two writes rather than one.
Caught by the integration tests going flakey.
Closes: #41 (closed)
Edited by Spindel 🕷 Ljungmark