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

jrgm/fxa-content-server,swatilk/fxa-content-server,ofer43211/fxa-content-server,chilts/fxa-content-server,ofer43211/fxa-content-server,npestana/fxa-content-server,swatilk/fxa-content-server,vladikoff/fxa-content-server,shane-tomlinson/fxa-content-server,TDA/fxa-two-factor-auth,mozilla/fxa-content-server,jpetto/fxa-content-server,shane-tomlinson/fxa-content-server,riadhchtara/fxa-password-manager,atiqueahmedziad/fxa-content-server,riadhchtara/fxa-content-server,mozilla/fxa-content-server,riadhchtara/fxa-password-manager,jpetto/fxa-content-server,chilts/fxa-content-server,chilts/fxa-content-server,jpetto/fxa-content-server,TDA/fxa-content-server,swatilk/fxa-content-server,TDA/fxa-two-factor-auth,ofer43211/fxa-content-server,riadhchtara/fxa-content-server,TDA/fxa-content-server,TDA/fxa-two-factor-auth,dannycoates/fxa-content-server,ReachingOut/fxa-content-server,riadhchtara/fxa-content-server,dannycoates/fxa-content-server,jrgm/fxa-content-server,shane-tomlinson/fxa-content-server,ReachingOut/fxa-content-server,riadhchtara/fxa-password-manager,vladikoff/fxa-content-server,vladikoff/fxa-content-server,npestana/fxa-content-server,dannycoates/fxa-content-server,atiqueahmedziad/fxa-content-server,ReachingOut/fxa-content-server,npestana/fxa-content-server,mozilla/fxa-content-server,TDA/fxa-content-server,atiqueahmedziad/fxa-content-server