Fix crash in debug when request body stream finishes after a >= 300 response head

[fabianfett]

Motivation

If a server responds with a status >= 300 while we are still uploading the request body, we ask the producer to pause the request body stream. The producer may still finish the stream, because it raced with the pause request. In this case we hit assert(head.status.code < 300) in HTTPRequestStateMachine.requestStreamFinished, which crashes debug builds.

Changes

• Remove the wrong assertion. Finishing the request body stream after a >= 300 response head is a valid state transition: we send the request .end and move into .endSent.
• Add a reproducer in HTTPRequestStateMachineTests that streams a body part, receives a 413 Payload Too Large head and then finishes the request body stream.

Result

A request body stream that finishes after the server responded with a status >= 300 no longer crashes in debug. Since the request .end is on the wire, the connection can be reused after the response ends.