Populates Issue's repository from URL when not in response.

Issues created from `Repository.get_issue()` have their `repository` field set to None.  This pull request manually sets the Issue's repository using the Issue's URL, when the repository has not already been set.

I've also modified the test in `tests.Issue.Issue.testAttributes` to verify that the repository is correctly set.  The modified test does fail if I remove my changes to the library.