[config] If "host" header is missing, fall back on "Host"
[config] If "host" header is missing, fall back on "Host"

Apiary converts header names to lowercase for Cloud Endpoints requests
but the endpoints webapp2 adapter doesn't. The header name as originally
described in rfc7230 is "Host", so falling back on that name should be
safe and there should be no need to expect any other case.

Bug: 805543
Change-Id: Ie14146ed37645eb1dd329235db0a9bbda2402437
Reviewed-on: https://chromium-review.googlesource.com/1056348
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Commit-Queue: smut <95deb7aa15d4c615299e47e7688551456568b1fb@google.com>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py