'title' is absent in JSON: {"number":1,"head":{"ref":"...' when wrapping `MkPull.java` with Pull.Smart()

Take a look:

```java
final Pull pull = new MkGithub().randomRepo().pulls().create("test", "testing", "master");
final String title = new Pull.Smart(pull).title();
```

And I got this:

```text
15:00:39.173 [main] INFO com.jcabi.github.mock.MkRepos -- repository jeff/cikNmfTTZ7tQOH7Iccav created by jeff
15:00:39.209 [main] INFO com.jcabi.github.mock.MkIssues -- issue #1 created in jeff/cikNmfTTZ7tQOH7Iccav by jeff: test

java.lang.IllegalStateException: 'title' is absent in JSON: {"number":1,"head":{"ref":"testing","label":"jeff:testing"},"base":{"ref":"master","label":"jeff:master"},"user":{"login":"jeff"},"comments":0}
```

Please, fix this.