Just test for the RTCPeerConnection property name

Currently, with the code as it is, including the `RTCPeerConnection` test in a Modernizr build causes the following exception to be thrown:

```
Uncaught TypeError: Object function RTCPeerConnection() { [native code] } has no method 'bind' 
```

Adding the third argument removes the exception, and the test works as expected on Chrome (which has `RTCPeerConnection`) as well as Safari and Firefox (which do not).