fix: catch errors in normalizeType instead of letting them throw

[einarpehrson]

Fixes #100.

normalizeType() no longer catches errors from content-type.parse() or media-typer.test(), unlike 1.x's tryNormalizeType. Since content-type@2 became more lenient (#95, building on jshttp/content-type#58) and can return an empty type string for a malformed Content-Type header (e.g. ; or an all-whitespace value), that empty string reaches media-typer.test(), which throws on falsy input rather than returning false.

This means a request with a malformed-but-present Content-Type header (combined with a Content-Length or Transfer-Encoding header) crashes typeis()/typeofrequest() with an uncaught TypeError, instead of returning false/null the way it does for a missing Content-Type header or other invalid media types.

This restores the try/catch so normalizeType degrades to null on any parse/test failure, matching the historical 1.x behavior, and adds a regression test.

Verified: the new test fails against current master with the exact TypeError: argument string is required from the linked issue, and the full suite (53 tests, was 52) passes with this fix applied. npm test and npm run lint both clean.