BUG/MINOR: cache: also cache absolute URIs

BUG/MINOR: cache: also cache absolute URIs

The recent changes to address URI issues mixed with the recent fix to
stop caching absolute URIs have caused the cache not to cache H2 requests
anymore since these ones come with a scheme and authority. Let's unbreak
this by using absolute URIs all the time, now that we keep host and
authority in sync. So what is done now is that if we have an authority,
we take the whole URI as it is as the cache key. This covers H2 and H1
absolute requests. If no authority is present (most H1 origin requests),
then we prepend "https://" and the Host header. The reason for https://
is that most of the time we don't care about the scheme, but since about
all H2 clients use this scheme, at least we can share the cache between
H1 and H2.

No backport is needed since the breakage only affects 2.1-dev.