Request body: avoid linking rb->buf to r->header_in. Code to reuse of r->request_body->buf in upstream module assumes it's dedicated buffer, hence after 1.3.9 (r4931) it might reuse r->header_in if client_body_in_file_only was set, resulting in original request corruption. It is considered to be safer to always create a dedicated buffer for rb->bufs to avoid such problems.

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