HTTP/2: enforce writing the sync request body buffer to file. The sync flag of HTTP/2 request body buffer is used when the size of request body is unknown or bigger than configured "client_body_buffer_size". In this case the buffer points to body data inside the global receive buffer that is used for reading all HTTP/2 connections in the worker process. Thus, when the sync flag is set, the buffer must be flushed to a temporary file, otherwise the request body data can be overwritten. Previously, the sync buffer wasn't flushed to a temporary file if the whole body was received in one DATA frame with the END_STREAM flag and wasn't copied into the HTTP/2 body preread buffer. As a result, the request body might be corrupted (ticket #1384). Now, setting r->request_body_in_file_only enforces writing the sync buffer to a temporary file in all cases.