fix request content-length header erasing after post redirect

[nwlunatic]

When performing a POST request with some data to server and it answers with redirect 303 See other with location of resource.
So when we are trying to handle redirect to that resource, we still have a Content-Length header left from POST. It cause send_headers method to try to write payload with no data but with length

    def send_headers(self, _sep=': ', _end='\r\n'):
        """Writes headers to a stream. Constructs payload writer."""
        ...
        elif self.length is not None:
            self.writer = self._write_length_payload(self.length)
        ...

Maybe this fix just fixing the symptom, but what I want is to pay attention at POST with data redirect problem

[asvetlov]

The change looks good but please add a test also.

[asvetlov]

Thanks

[lock]

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