The upcast element to attribute fails to convert when upcastAlso is defined as a function

[jodator]

Found out after ckeditor/ckeditor5-basic-styles#94 got merged. The prepareToAttribute() converter exits when in a config.view only a name of an element was defined. That check didn't account for the possibility of a function being provided. This would wrongly mark such definitions as with an only name defined and wasn't converting an attribute properly.

This fix is simple and only needs some tests to be written.