fix: prevent Content-Type header from being set to 'false'

[veeceey]

Fixes #7034

When res.set('Content-Type', value) is called with a value that mime.contentType() can't resolve (returns false), the header ends up being set to the literal string "false".

This happens because mime.contentType() returns false for unrecognized types, and that false gets passed straight to setHeader(), which coerces it to a string.

// before this fix:
res.set('Content-Type', 'custom-type');
// => Content-Type: false

The fix is a one-liner — fall back to the original value when mime.contentType() returns false. This is consistent with how res.type() already handles the same case (it falls back to 'application/octet-stream').

// after:
res.set('Content-Type', 'custom-type');
// => Content-Type: custom-type

Added a test to cover this edge case. Full test suite passes (1247 tests).