A new request should always have a new content-length (#16667)

* A new request should always have a new content-length

In [my last PR](https://github.com/nim-lang/Nim/pull/16618) I made a mistake by assuming that the client.headers were cleared on every request, like the Python version. So, due to the fact that Nim keeps the client headers, we need to clear the Content-Length header on each request (which makes sense because you almost never want to use the same Content-Length twice, but you may want to reuse other headers)

* Move content-length to newHeaders instead of in the global client headers

* Use single backticks

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