Ensure new options values are always properly typed.

Previously values read from pants.ini were always strings.

Amend the relevant unit test to detect a regression of this.

Testing Done:
Fixed the test, then ran it before and after the fix.

ci baking.

Reviewed at https://rbcommons.com/s/twitter/r/1175/
Sync of https://github.com/pantsbuild/pants/commit/be7961a4eba1f01dccbe21fb5a5a80138c5d0d16