Add CapturingTransport#captureRequestsAndClear

We use the pattern of 

``` java
CapturingTransport.CapturedRequest[] capturedRequests = transport.capturedRequests();
transport.clear()
```

where `transport` is a `CapturingTransport` throughout our test suite. This pattern could be simplified by adding a dedicated `CapturingTransport#captureRequestsAndClear` method.
