fix: generic type error of RouteShorthandMethod

[XHFkindergarten]

closes #280

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

Could you add a test for this type change? We use tsd.

[XHFkindergarten]

Could you add a test for this type change? We use tsd.

done，I think a default type check is enough.

[mcollina]

lgtm