Fix bug in Sisimai::Message->tidy() method

[azumakuniyuki]

• Fix typo in Sisimai::RFC1894 X-Actual-Recipient
• Fix bug in code for checking error messages at Sisimai::Message->tidy
• More strictly test for Sisimai::Message->tidy() at t/040-message.t
• Compare the value of the parameter in Diagnostic-Code: field with the upper-cased string

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 93.33333% with 3 lines in your changes missing coverage. Please review.

Project coverage is 85.62%. Comparing base (69b277b) to head (6ceda72).

Files	Patch %	Lines
lib/Sisimai/Message.pm	95.23%	0 Missing and 2 partials :warning:
lib/Sisimai/Lhost/Amavis.pm	0.00%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 5-stable #540 +/- ## ============================================ + Coverage 85.58% 85.62% +0.04% ============================================ Files 135 135 Lines 6591 6597 +6 Branches 1932 1932 ============================================ + Hits 5641 5649 +8 + Misses 304 302 -2 Partials 646 646 ```

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