getSelectionCustomInlineStyle does not return values for multiple styles

[Nikita Seleckis]

E.g. if is set 'FONTSIZE' for one part of text, and same style but with different value for another, and we select the whole text, then getSelectionCustomInlineStyle returns FONTSIZE: undefined. Seams it should return an array of styles, like: ["fontsize-30", "fontsize-22"].

This can be reproduced in https://github.com/jpuri/react-draft-wysiwyg

I'm posting it here, because react-draft-wysiwyg is utilizing this function, which should do its job better. :)