httppeer: don't send empty Vary request header
As part of writing test-http-bad-server.t, I noticed that some
requests include an empty Vary HTTP request header.
The Vary HTTP request header indicates which headers should be taken
into account when determining if a cached response can be used. It also
accepts the special value of "*".
The previous code unconditionally added a Vary header. This could lead
to an empty header value. While I don't believe this violates the HTTP
spec, this is weird and just wastes bytes. So this patch changes
behavior to only send a Vary header when it has a value.
Some low-level wire protocol byte reporting tests changed. In some
cases, the exact point of data termination changed. However, the
behavior being tested - that clients react when the connection is
closed in the middle of an HTTP request line or header - remains
unchanged.
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