By default, we check the length of response body matches
content-length.  For HEAD request, this is not necessarily true, so we
sniff request method, and if it is HEAD, make sure that response body
length is 0.  But this does not work for HTTP Upgrade, since
nghttp2_session_upgrade() has no parameter to tell the request method
was HEAD.  This commit disables this response body length validation
for the stream upgraded by HTTP Upgrade.  We will add new version of
nghttp2_session_upgrade with the parameter to pass the request method
information so that we can handle this situation properly.