Upstream: fixed store/cache of unfinished responses. In case of upstream eof, only responses with u->pipe->length == -1 are now cached/stored. This ensures that unfinished chunked responses are not cached. Note well - previously used checks for u->headers_in.content_length_n are preserved. This provides an additional level of protection if protol data disagree with Content-Length header provided (e.g., a FastCGI response is sent with wrong Content-Length, or an incomple SCGI or uwsgi response), as well as protects from storing of responses to HEAD requests. This should be reconsidered if we'll consider caching of responses to HEAD requests.