Style override only works for attributes

[Garry Yao]

1. Prepare the editor with the following configuration, to avoid nested font family style:

   config.font_style =
   	{
   		element		: 'span',
   		styles		: { 'font-family' : '#(family)' },
   		overrides	: [ { element : 'span', styles : { 'font-family' : null } } ]
   	};
   
   

2. Load the the following content and selection:

   <span style="font-family:lucida sans unicode,lucida grande,sans-serif;">some [sample] text</span>
   

3. Open font name combo and click on "Aria";

• Actual Result:

  <p>
  	<span style="font-family:lucida sans unicode,lucida grande,sans-serif;">some <span style="font-family:arial,helvetica,sans-serif;">sample</span> text</span></p>
  

• Expected Result:

  <p>
  	<span style="font-family:lucida sans unicode,lucida grande,sans-serif;">some </span><span style="font-family:arial,helvetica,sans-serif;">sample</span><span style="font-family:lucida sans unicode,lucida grande,sans-serif;"> text</span></p>
  

[Jakub Ś]

This ticket can be considered as part of one large issue mentioned in #5503.