Support different agent for different protocols

[Tom910]

Problem

const customAgent = new https.Agent({ keepAlive: true })
function request(url, method) {
	return new Promise(resolve => {
	  request[method](url)
	      .agent(customAgent)
	      .end(function(){
	       ....
	       })
	}
}

request('https://github.com') // > work fine
request('http://github.com') // > dosen't work

The current api does not allow to transfer different HTTP agents for different protocols. And it is necessary to parse the URL in the request function. And expose different agents.

Possible Solution

got way - https://github.com/sindresorhus/got#agent

got('https://sindresorhus.com', {
	agent: {
		http: new HttpAgent(),
		https: new HttpsAgent()
	}
});

node-fetch way - https://github.com/bitinn/node-fetch#custom-agent

const customAgent = function(_parsedURL) {
    if (_parsedURL.protocol == 'http:') {
        return httpAgent;
    } else {
        return httpsAgent;
    }
}