Attaching a database (created in SQL Server 2012) to an instance of SQL Server 2014 without upgrading it
It is not possible for that upgrade to not happen. There are changes required for the database to be attached and usable on newer versions. 

This is under-the-hood stuff like internal references and pointer formats, the exact internal changes from version to version aren't documented. See here for some detail if only to satisfy your curiosity, but I would never risk trying to edit those bits directly.

It is possible to not change the compatibility level of the database, however you would still not be able to take a backup of that database and restore it to another SQL Server 2012 instance.

Because of differences in server/database versions your production and development servers should be on the same versions to really get the full benefit of testing in dev. If you aren't going to upgrade production soon, then you could downgrade dev.