Bug 959708 - [marionette-plugin-forms] setValue fails if element has not been seen before r=jugglinmike

I thought error was caused by empty `[type]` initially (since error message was on `type.trim()`) that's why I added tests for inputs with missing `[type]`.

I believe this change will make errors easier to spot. I spent a good time trying to figure out why my tests was failing - it was caused by trying to set the value of an element that didn't existed on the document.