Fix exception in helper on asset check in via mobile.

On an instance of redmine at a job of mine with current plugin version, an exception was raised when requesting `/equipment_assets/3/asset_check_ins/new` via mobile:
https://gist.github.com/49429169c688c3033771

I did a quick fix for this without really understanding, why `options[:pos][0]` is `Fixnum`, here, but I think, this not breaking anything.