HTTP/1.1: normalize Host header

HTTP/1.1: normalize Host header

When absolute-URI is provided Host header should be ignored. However some
code still uses Host directly so normalize it using the previously
sanitized URL authority value before doing any further request processing.

For now preserve the case where Host is completely absent. That matters
to the CVE-2009-0801 protection.

This also has the desirable side effect of removing multiple or duplicate
Host header entries.