Mocking of  header method

I'm trying to use ``mock-req`

Initialization in Unit tests:

```
MockReq = require('mock-req'),
var req = new MockReq();
// req.header = function(){} // with this line it does not work also
```

Testable code:

```
if (req.header('content-range')) {
```

I'm getting message:

```
TypeError: Object #<MockIncomingMessage> has no method 'header'
```
