Allow custom Content-Type for Buffer/Stream returned from fn.

[amio]

Currently the custom Content-Type in fn (for example):

const fetch = require('node-fetch')

module.exports = function (req, res) {
  return fetch('https://httpbin.org/ip').then(r => {
    res.setHeader('Content-Type', r.headers.get('Content-Type'))
    return r.body
  })
}

would be overridden by micro.send() method. This PR fix this :)

[leo]

Please add an empty line after this statement.

[amio]

@leo PR updated 💪

[leo]

@amio 😘