use default interpolator instead of return original translationId

if not define translationId

before:
$filter("translate")("Name: {{name}}", {name: "User"}) // "Name: {{name}}"

after:
$filter("translate")("Name: {{name}}", {name: "User"}) // "Name: User"

Add test case for @knalli asking.
I'm not sure how to refer the #716 , so I send a new pull request.