SPDY spec does not allow multiple empty header values in one header
field. This change makes out-going framer ignore such empty header
value if there is non-empty header value with the same name.

This change fixes the bug that spdylay_frame_unpack_nv does not check
the size of header value if it is the last value in NULL separated
list.

The RST_STREAM will be issued in spdy_data_read_callback.

spdy_data_read_callback in SpdyDownstreamConnection calls
SpdyUpstream::resume_read() which submits WINDOW_UPDATE, but after
that they are not call SpdyUpstream::send(). This means that if no
pending outgoing data in upstream, then WINDOW_UPDATE is blocked until
SpdyUpstream::send() from somewhere. This change adds
SpdyUpstream::send() to resume_read() so that WINDOW_UPDATE is not
blocked.

This change fixes upstream RST_STREAM is blocked until
SpdyUpstream::send() is called. Now downstream REFUSED_STREAM is
propagated to upstream client so that client can reset request.  The
RST_STREAM error code when downstream went wrong is changed from
CANCEL to INTERNAL_ERROR.

Use --backend-spdy-no-tls to disable TLS on backend SPDY connection.
The SPDY protocol used there must be configured by
--backend-spdy-proto option.

Surprisingly, field_set & UF_USERINFO is nonzero even if userinfo
component is empty string.

Now we set Downstream::set_response_connection_close(true) for
tunneled connections. Also call
Upstream::on_downstream_body_complete() callback when setting
MSG_COMPLETE in SpdySession when RST_STREAM is caught.  Clean up EOF
handling in https_downstream_readcb.