Fix an issue with NTLM authentication

NTLM authentication sends a challenge and response which translate to www-authenticate fields which are concated into a single string. When you send the response back proxied you need to split the string and send it back as an array. You will still need to create a keepalive agent to use this. So I used the library agentkeepalive like so..

To actually use this code a relevant code snippet might be:
```js

var httpProxy = require('http-proxy');
var Agent = require('agentkeepalive');

var agent =  new Agent({
  maxSockets: 100,
  keepAlive :true,
  maxFreeSockets: 10,
  keepAliveMsecs:1000,
  timeout: 60000,
  keepAliveTimeout: 30000 // free socket keepalive for 30 seconds
});
var proxy = httpProxy.createProxyServer({ target:myserver,agent:agent});
```
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