Fix MultiJson when direct streaming is disabled

[dawust]

When writing data using PostData.MultiJson, the FinishStream method is incorrectly duplicating data into the same buffer when disableDirectStreaming is set to true. This happens because BufferIfNeeded modifies writableStream by reference to use a newly created MemoryStream buffer, but FinishStream later uses the same writableStream (now the buffer) for copying, resulting in self-copying and data duplication.

I think, the issue can be resolved by introducing a local variable stream to hold the current writing stream, ensuring that the original writableStream remains distinct from the buffer. The local stream is used for writing data, while the original writableStream is used correctly in FinishStream.

[flobernd]

cc @stevejgordon Seems like a regression from our stream refactoring. Let me know if I should take care, or if you want to address it.

[stevejgordon]

@flobernd Yep, it sounds like the refactor missed something here. I'm unlikely to get to this until January. If you have capacity to take a look, that would be much appreciated.

[flobernd]

@stevejgordon Sure 🙂 Will have a look on Monday.

Thanks @dawust for creating a PR!