Add 100 Continue support

[JoshCap20]

Category: HTTP/HTTPS Proxy RFC Compliance

Problem

RFC 7231 §5.1.1 defines the Expect: 100-continue mechanism. When a client sends this header, it expects a 100 Continue interim response before sending the request body. The proxy currently ignores this header, which can cause:

• Clients waiting indefinitely for a 100 Continue that never comes
• Timeouts on large uploads

Current State

The Expect header is passed through to upstream without any proxy-level handling. The proxy reads the body immediately regardless of Expect: 100-continue.

Proposed Solution

1. Detect Expect: 100-continue header
2. Before reading the body, send HTTP/1.1 100 Continue\r\n\r\n to the client
3. Then proceed with reading the body
4. If upstream rejects (e.g., 417 Expectation Failed), forward that response instead

Priority

Low — Most HTTP clients handle the absence of 100 Continue gracefully (they send the body after a timeout). Important for correctness with large uploads.