Fix incorrect locale pref reading

Reopen #44 (and original #42). @PinZhang, the test changes causes errors. Seems like the "locale" field is undefined after switching to getLocalized ?

```
console.error: firefox-personalization-study: 
  fail:
  unexpected payload data - [
    "hasSurveyInterests",
    "installDate",
    "interests",
    "nytUserData",
    "nytVisits",
    "payloadDate",
    "prefs",
    "source",
    "tldCounter",
    "updateDate",
    "uuid",
    "version"
] == "hasSurveyInterests,installDate,interests,locale,nytUserData,nytVisits,payloadDate,prefs,source,tldCounter,updateDate,uuid,version"
console.trace: firefox-personalization-study: 
_ecated/unit-test.js 87 fail
_/sdk/test/assert.js 88 fail
_/sdk/test/assert.js 124 equal
_dy/tests/helpers.js 21 do_check_eq
_dy/tests/helpers.js 43 isIdentical
_/test-Dispatcher.js 150 test__Dispatch/</testPayload
_/test-Dispatcher.js 159 test__Dispatch/</<
_s/sdk/test/httpd.js 2220 ServerHandler.prototype.handleResponse
_s/sdk/test/httpd.js 1139 Connection.prototype.process
_s/sdk/test/httpd.js 1587 RequestReader.prototype._handleResponse
_s/sdk/test/httpd.js 1435 RequestReader.prototype._processBody
_s/sdk/test/httpd.js 1304 RequestReader.prototype.onInputStreamReady
```