Log unknown severities and String or nil tags instead of raising

[PetrHeinz]

logger.unknown("x"), logger.add(nil, "x") and logger.add(99, "x") raise KeyError: key not found: "ANY" from inside the formatter, so a call meant to log something crashes the caller instead. Ruby's Logger labels UNKNOWN, a nil severity and anything above it "ANY", and Formatter::SEVERITY_MAP only knows "UNKNOWN". A user field tags: holding a String or nil raises TypeError from Array#concat the same way. The red-team probe against 0.1.20 printed unknown: KeyError: key not found: "ANY" for all three severity calls.

• SEVERITY_MAP maps "ANY" to :unknown, and a label it doesn't know falls back to :unknown instead of raising, just as Ruby's Logger#format_severity falls back to "ANY".
• tag: and tags: go through Array(...), so a single String becomes a one-element list, and nil tags are skipped.

Behaviour and compatibility:

• The calls above now log a line with level unknown, or with the given tags. Documented calls are unchanged.
• tag: nil and nils inside a tags: array used to be sent as null tags. They are now left out.
• tag: holding an Array is now flattened into the tags instead of being nested as a single element.

Targets the logtail 0.1.21 patch release. No dependencies on the other open PRs.

The first commit only adds the tests and is expected to fail on CI; the fix follows in the next commit.

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