Handle selects (etc) with numeric keys

[jasonvarga]

When select fields (or radio/checkboxes/button_group fields) use numeric options, like this...

myfield:
  field:
    type: select
    options:
      1: One
      2: Two

...the fieldtype ends up saving the value as a string (javascript objects can't have numeric keys), so you end up with this:

myfield: '1'

Then in #4483, it was made so that if the value didn't match an option exactly, it would use itself as the value and label.
The string version of the number does not exactly equal the option in this case.

This PR makes it so that integer strings get treated as integers.

ie. both these work

myfield: '1'

myfield: 1

Floats get treated as strings though, since you can't have a float as a key in PHP. They'll be strings, even if there's a real float in the data.

myfield:
  field:
    type: select
    options:
      1.5: One point five  # technically '1.5'
      2.5: Two point five  # and '2.5'

ie. both these work:

myfield: '2.5'

myfield: 2.5

Fixes #4574