BUG EditableFormHeading doesn't properly handle numeric values

[muskie9]

There seems to be a bug with EditableFormHeading where passing a value such as 2016 is actually rendered as the generated field name, for example something like EditableTextField_f157b. I've tracked this back to the way HeaderField works in framework's HeaderField and how EditableFormHeading passes the values to the HeaderField created.

I've traced this as far back as framework 3.1.x-dev with userforms 3.0.x-dev. I haven't completely uncovered the underlying issue why integer values aren't rendered are affected, but my fix seems to work with both integers and strings. I'll have a PR against the 3.0.x-dev branch and can re-submit if needed against another branch if that is too far back. I may need some help with tests if those are required for the PR but I didn't see any existing tests for this particular class.