Fix #380: Require a javascript content-type.

I want to double-check that we want to omit the `*/ecmascript` content types defined by http://www.rfc-editor.org/rfc/rfc4329.txt. Nothing else from http://www.whatwg.org/specs/web-apps/current-work/multipage/scripting.html#scriptingLanguages looks like a good idea.