Use `urljoin()` in assertRedirects instead of string concatenation

[migurski]

String concatenation does not correctly detect redirects across domains. Using urlparse.urljoin() accounts for the basic case already covered by simple concatenation, as well as redirects across domains.

[jcass77]

@migurski, it looks like the Python 3 tests are failing for this PR.

[migurski]

I updated the import to work with both Python 2 and Python 3.

[jcomo]

There were conflicts with the merge due to hold old this PR is. I cherry-picked out your changes and merged here: https://github.com/jarus/flask-testing/commit/70a4bda0d888b2e2850454527edeacf1e939944d.

[migurski]

Thanks