Don't remove parameters with empty values.

Sometimes things need to be set to nil or an empty string.

In our case we're using Weary to update a central user database. Somebody who loses a job for instance has to have their title and department changed to nil or an empty string. But these params were being removed.

I can't think of any reason empty params should be removed and all the tests passed without the code that removes them so it seems like there's no reason to have that functionality.