Upstream: fixed store/cache of unfinished responses.

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.