return this if the proxy uri is null

Sometimes production env is different from development env.

In production config
```
config = {
  proxy: null
};
```

In development config
```
config = {
  proxy: 'http://10.1.10.200:3128'
};
```

It is nice to have this api syntax.
```
var config = require('config');
request
    .get('http://www.google.com')
    .proxy(config.proxy)
    .end(function (res) {});
```

otherwise, i need to write something like this

```
var config = require('config');
req = request.get('http://www.google.com')
if (config.proxy) {
  req = req.proxy(config.proxy);
}
req.end(function (res) {});
```

more ugly