GetRequest should use the requestKey

I have been trying to do a request synchronously using `get()` but it failed because the `GetRequest` didn't have a `requestKey`.

This pull request also includes a sample activity to reproduce the issue.

Also check the `fetch()` method:

```
  public void fetch() {
    String requestKey = createKey(uri, resourceId, options, transformations);
    Request request =
        new FetchRequest(picasso, uri, resourceId, options, transformations, skipMemoryCache);
    picasso.enqueueAndSubmit(request);
  }
```

`requestKey` is created but never used.