chore(tests): Add tests to ensure the correct client.json is returned under a variety of circumstances.

* all uppercase language
* all lowercase language
* uppercase region
* uppercase language
* multiple regions supported, only language specified
* unsupported region returns root language
* no Accept-Language header sent
* Three part locale

issue #979
Merge pull request #1074 from mozilla/issue-979-test-locale-handling

chore(tests): Add tests to ensure the correct client.json is returned under a variety of circumstances.