test: add edge case coverage for createError factory

[mahmoodhamdi]

What

Added 16 new test cases covering several untested edge cases in the createError factory and named constructors.

Why

I use http-errors extensively in my Express middleware and noticed some scenarios that lack test coverage while reading through the source. These tests document and verify existing behavior for edge cases that users commonly encounter.

New test cases

createError() with no arguments

• Defaults to status 500, message "Internal Server Error", expose false

err.statusCode extraction

• When an error has statusCode but not status, statusCode is used
• When both status and statusCode exist, status takes precedence

expose property

• Verified true for 4xx errors via both factory and named constructors
• Verified false for 5xx errors via both factory and named constructors
• Tests multiple status codes in each category (400, 404, 499, 500, 503, 599)

Custom headers via properties

• Headers from the properties object are correctly attached to the error
• Headers are undefined by default

Argument type validation

• TypeError thrown for boolean arguments
• TypeError thrown for function arguments

Named constructors with custom messages

• Client error (BadRequest) with custom message
• Server error (ServiceUnavailable) with custom message
• null message falls back to default status message

Testing

68 passing (7ms)
1 pending

All existing tests continue to pass. Linting passes.