Fix MockServer base url bug

I thought about introducing an immutable internal object to represent the current state of MockService but decided that was too complex, and instead opted for a simple fix. Tests are updated and green.

I still think this object needs some work however, as it continues to have unexpected behaviour due to shared state. For example:
```
var mock1 = Pact.mockService({consumerName: 'client', port: 1234, providerName: 'server', pactDir: 'spec/pacts'});
mock1.write(); //makes request to localhost:1234
var mock2 = Pact.mockService({consumerName: 'client', port: 4321, providerName: 'server', pactDir: 'spec/pacts'});
mock2.write(); //makes request to localhost:4321
mock1.write(); //makes request to localhost:4321 - this is unexpected
```