Refactor client test framework now that Requester is merged

[flotter]

@flotter and @anpep have an outstanding task to refactor the client test framework to better utilise the Requester interface (#310). At the moment the test framework cannot use Requester Do() due to client.response type not used in the response body. We should also ensure all areas are properly covered.

[flotter]

Comment from Ben:

func (client *Client) SetDoer(d doer) {
	client.doer = d
	client.Requester().(*defaultRequester).doer = d

Should this SetDoer method be moved to a test method/function? I believe external packages can't actually call it anyway, as the doer interface isn't exported. But happy to leave this to another PR if you prefer.

[flotter]

Comment from Ben:

It looks like we're never setting the baseURL field -- oversight? We do use it in dispatch (somewhat surprised there's not a test for this -- we should probably add one).