delete request should contain content-length in header

[jaredleechn]

reproduce

fetch('/somePath', { method: 'delete' });

• make a delete with no body
• there's no content-length in the request header

expect

The delete request with no body should contains header content-length: 0 like other verbs

[jaredleechn]

the bug fail the hasBody check of a http request from package type-is, which is dependent by koa.js