merge with default options

This should help in situations like this:

```javascript
var r = require('request');

var e = r.defaults({
  headers: { Authentication: 'Bearer 123abc' }
});

var f = e({
  url: 'http://foo',
  headers: { 'X-Content-Length': 10000 }
});

console.log('%j', f.headers);
// {"Authentication":"Bearer 123abc","X-Content-Length":10000,"host":"foo"}
```