Tidy up some .pm and .t

[yoshikazusawa]

Rework #203.

d231eb5 Fix small path correction in CONTRIBUTING.md.
5228112 I applied perltidy to some .pm and .t that is not to affect with other open PRs.
bfa7cc0 Also made some manual adjustments to tab characters in comments.

The difference from #203 is the modified files were narrowed down using the following command (with refspec added to remotes/pr/).

% comm -23 <(git ls-files | sort) <(echo -n "8,16,23,25,35,93,99,117,122,141" | xargs -d, -I% sh -c 'git diff --name-only master...remotes/pr/%/head' | sort | uniq) | grep -E -e 'lib/' -e 't/.*\.t$'
lib/HTTP/Headers/Auth.pm
lib/HTTP/Headers/ETag.pm
lib/HTTP/Headers/Util.pm
t/common-req.t
t/headers-auth.t
t/headers-etag.t
t/headers-util.t
t/lib/Secret.pm
t/message-brotli.t
t/message-charset.t
t/message-decode-brotlibomb.t
t/message-decode-bzipbomb.t
t/message-decode-xml.t
t/message-decode-zipbomb.t
t/message-old.t
t/request_type_with_data.t
t/response.t
t/status-old.t
t/status.t