Before this change, int() on a non-numeric Content-Length raised an
unhandled ValueError, crashing the request handler. There was also no
upper bound on how much memory a POST body could consume.
After this change:
- Non-numeric or missing Content-Length returns HTTP 400.
- Negative Content-Length returns HTTP 400.
- Bodies declared larger than 1 MiB (_MAX_BODY_BYTES) return HTTP 413,
matching the cap already in supervise_server.py.
Closes#138