ckeditor - setting styles not working; editor adds extra apostrophes

So, it seems like the editor adds additional quotes to the class="" definition, when you define styles.

This is how my config.js:

/**
 * @license Copyright (c) 2003-2013, CKSource - Frederico Knabben. All rights reserved.
 * For licensing, see LICENSE.md or https://ckeditor.com/license
 */

 // additional styles
CKEDITOR.stylesSet.add( 'sla_styles', [
    // Block-level styles.

    // Inline Styles
    { name: 'weißer Text', element: 'span', styles: { 'color': '#ffffff' } }
]);

CKEDITOR.editorConfig = function( config ) {
    // Define changes to default configuration here.
    // For the complete reference:
    // https://docs.ckeditor.com/#!/api/CKEDITOR.config

    // The toolbar groups arrangement, optimized for two toolbar rows.
    config.toolbarGroups = [
        { name: 'clipboard',   groups: [ 'clipboard', 'undo' ] },
        { name: 'editing',     groups: [ 'find', 'selection', 'spellchecker' ] },
        { name: 'links' },
        { name: 'insert' },
        { name: 'forms' },
        { name: 'tools' },
        { name: 'document',    groups: [ 'mode', 'document', 'doctools' ] },
        { name: 'others' },
        '/',
        { name: 'basicstyles', groups: [ 'basicstyles', 'cleanup' ] },
        { name: 'paragraph',   groups: [ 'list', 'indent', 'blocks', 'align', 'bidi' ] },
        { name: 'styles' },
        { name: 'colors' },
        { name: 'about' }
    ];

    // Remove some buttons, provided by the standard plugins, which we don't
    // need to have in the Standard(s) toolbar.
    config.removeButtons = 'Underline,Subscript,Superscript';

    // Se the most common block elements.
    config.format_tags = 'p;h1;h2;h3;h4;h5;h6';

    // Make dialogs simpler.
    config.removeDialogTabs = 'image:advanced;link:advanced';

    // eigene Styles
    config.stylesSet = 'sla_styles';
};

I dont know if it is a bug, but maybe someone can suggest a workaround for this or tell me what I am doing wrong here?

Thanks in advance!