__isset magic method doesn't check for main table

I ran in an issue where form fields created with the form builder didn't pre-populate correctly. 

After some debugging I found out that polyglot does only check the *_lang table if there are any translated attributes. Because e.g. checkbox values don't need translations, they shouldn't bee stored in the *_lang table, but instead stay in the 'normal' table. 

My pull request fixes this issue. Polyglot now checks if there's a translated version of this attribute first and if not, just delegates to the parent __isset method. Like that we achieve the desired behavior.