Inspect `content` in assertion message of '#each' helper

I improved assetion message to inspect `content` of `#each` helper.

before:
> You passed [object Object] (wrapped in (generated ontology.index controller))

after:
> You passed '{}' (wrapped in (generated ontology.index controller))

"[object Object]" should be inspected for kindness of debugging.
Merge pull request #3866 from tricknotes/improve-assertion-message

Inspect `content` in assertion message of '#each' helper