Comment object has no "by" field

I encountered a comment at the bottom of a page, but the commenter was not displayed. I discovered that the comment document has no "by" field which the mustache template needs. (Also missing is `gravatar_url`.)

Is a good fix to add this to the validator? Not confident with the Pages code, I wanted to ask first.

(Also of course there is presumably some client code with some kind of bug.)
