Investigate interop multiplexing test failed for server

[nhorman]

The quic-interop-runner multiplexing test failed. The logs indicate that, while all files were served by the server and downloaded by the client, there was a file size mismatch (32 bytes on the server vs 0 bytes on the client) on one of the sent files.

Checking the logs it appears the short file (which is randomly generated by the test harness) was named bsyynqphdv, and the request was sent, but the server interpreted the name as bsyynqp (i.e. truncated), and as such could not find the file.

Investigation needed to determine why the file was truncated and fix it. Likely a server application issue

[Sashan]

failure in interop, I think we should fix the code to pass this test.