Rewrite ckan.logic.converters.convert_to_extras 

This branch introduces a single change:

* Rewrite the `convert_to_extras` function so it behaves in a way that doesn't interfere with other extras and complements the related convert_from_extras function.

I notice there is no test coverage for these.

Fixes #2150.
You refactored line 16 into new lines 15-18 and TBH I find the original easier to understand.