fix(server): Fix IE9 not showing any content.

* JSON responses were being sent with a content type of `utf8` instead of `utf-8`. `utf8` is invalid and IE would not parse the JSON. This resulted in neither config nor translations being loaded.

fixes #1095
Merge pull request #1097 from mozilla/issue-1095-ie9-blank

fix(server): `utf8` is not a valid content-type, `utf-8` is.